* The Mustang began life powered by the American Allison engine. While the Allison would not prove the best match to the aircraft, a large number of Allison-powered Mustangs were built and gave good service.
* North American Aircraft (NAA) was founded in 1934, with 75 employees under President James H. "Dutch" Kindelberger, previously of Douglas Aircraft. NAA was an outgrowth of North American, an aviation-oriented holding and investment organization that had been established in 1928. The NAA offshoot was created with General Motors backing, and built a plant in Inglewood, a suburb of Los Angeles, California. NAA's first major aircraft designs were a series of trainers that culminated in the popular AT-6 Texan.
By 1937, the governments of Britain and France were becoming increasingly alarmed by German rearmament, and were not only ramping up their own production of aircraft, but looking to American sources as well. In 1938, the British placed a large order for North American trainers, resulting in substantial business for the company.
By 1939, the British and French were at war with the Nazis and were scrambling to get their hands on combat aircraft. The only fighters the Americans had available in 1939 and 1940 that seemed to be of any use were the Curtiss P-40 Warhawk and Bell P-39 Airacobra, and neither of them were notably impressive. The British still purchased a large number of P-40s, which proved serviceable if not more than that, and a small quantity of P-39s, which the British rejected after evaluation.
Since Curtiss was heavily committed to building the P-40 for the United States Army Air Corps (USAAC) and had little production capacity to spare, a delegation sent by the British government requested that North American produce P-40s under license from Curtiss for the Royal Air Force (RAF). NAA responded with a proposal, submitted to the British delegation in January 1940, that the company instead build an entirely new machine of their own design for the British, based on the Allison V-1710 water-cooled 12-cylinder inline vee engine used in the P-40, but with better range and performance. Obviously, a proprietary design would mean more profits for North American, but there was more than greed as a motive. The P-40 design had roots going back to 1933, and North American engineers were hardly being egotistical to suggest they could build something with better performance, as well as easier to manufacture, using the latest technology. The British delegation was favorable to the idea.
NAA mailed the British delegation in New York drawings of a design concept for the new aircraft in early May 1940, and on 29 May the British awarded a contract to North American for the "NA-73X" fighter, where "NA" of course stood for "North American". The contract specified initial prototype delivery in January 1941, and completion of the initial order by September 1941. NAA gave the production price of their new aircraft as $50,000 USD in 1941 dollars. After discussions, NAA and the British delegation agreed to an informal schedule for delivery of the first prototype within 120 days. This apparently wasn't a rigid deadline, since the formal contract gave NAA breathing room -- but in any case, the NAA design team threw itself into the task with a vengeance.
* The chief designer for the NA-73X was Edgar Schmued, who had been born in Germany, leaving the country for Brazil in 1925, and then emigrated to the US in 1930. He worked closely with aerodynamics specialist Edward Horkey. The team was under the overall direction of NAA Chief Engineer Ray Rice.
Since NAA had never built a fighter, at the request of the British NAA Vice-President Leland Atwood consulted with Curtiss, obtaining information on the advanced Curtiss "XP-46" fighter then under development. There were similarities between the design of the NA-73X and XP-46 that would eventually lead Curtiss engineers to accuse NAA of plagiarism, but NAA had presented their concept to the British before blueprints were obtained from Curtiss, and XP-46 work significantly lagged NA-73X development in any case. Some NAA advocates would later claim that the XP-46 provided NAA with little more than a bad example -- but the matter is academic since the XP-46 program was abandoned in 1941, after the construction of two prototypes. The NA-73X and its descendants also had a general layout similar to that of the Messerschmitt Bf 109, which would lead to absurd rumors that the design of the NAA aircraft was derived from the German fighter.
Although NAA appears to have paid little attention to the XP-46, the NA-73X design team clearly made use of information provided by the US National Advisory Committee for Aeronautics (NACA) on the "laminar" wing. Contemporary wing design featured a wing cross-section with maximum thickness about a fifth of the way across the wing from the leading edge, with most of the curvature, or "camber", on the top of the wing. The laminar flow wing, in contrast, was thickest halfway across the wing from the leading edge, and had almost as much camber on top as on the bottom. This scheme reduced turbulent flow across the wing, cutting down drag to increase both speed and range. The penalty was slightly inferior low-speed performance. The NA-73X's wing featured large flaps to reduce take-off and landing run.
Everything was done to make the NA-73X as aerodynamically clean as possible. The radiator and oil cooler were unconventionally placed on the bottom of the fuselage behind the cockpit, with an airflow exit flap towards the tailwheel. This arrangement provided aerodynamic benefits, at the cost of some additional pipe and duct work. The resulting aircraft was smooth and attractive. A carburetor intake tube above the prop spinner was one of the few discordant elements. In the NA-73X and a few following aircraft, the entrance to this tube was positioned well back from the prop spinner, but NAA engineers were to find out the hard way that the entrance needed to be moved up to just behind the propeller. The propeller itself was a three-bladed variable pitch type, provided by Curtiss.
NAA engineers did all they could to combine smooth contours with simplicity of form and underlying structure, with an eye towards ease of manufacture. The cowling, though close-fitting, was easy to remove, and the engine could be conveniently inspected and removed. Construction was almost entirely of metal, mostly aluminum alloy, except for fabric-covered rudders and elevators. The fuselage was in three sections that could be pulled apart after undoing bolts. Each wing was organized as an inner and outer section. The fuselage and wing sections were designed to be assembled as functional units, and then mated together at final assembly, substantially simplifying manufacture.
Provision was made for mounting two 12.7-millimeter (0.50-caliber) Browning machine guns in the nose, positioned under the engine in a staggered fashion to allow accommodation of the ammunition boxes, and firing through the propeller arc using a synchronizer mechanism. Primary armament, however, was to be one 12.7-millimeter and two 7.62-millimeter (0.30-caliber) Browning machine guns mounted in the center of each wing. The wing was so thin that the guns could not be positioned upright, being placed so that they almost rested on their sides -- an arrangement would lead to difficulties in service. The machine guns were also offset vertically, with the two outer guns firing out the leading edge of the wing and the middle gun firing just below the leading edge. A bomb or other store could be carried on a single pylon under each wing.
The cockpit was relatively roomy, and well laid out by the standards of the time, with a "razorback" canopy configuration. The pilot's seat had 8-millimeter-thick armor behind it, and the front of the canopy was armor glass. The canopy had a hinged panel on the left and a hinged top for entrance and exit. While the panel could not be opened in flight, it could be jettisoned in an emergency.
The main landing gear were simple and robust. They were hinged in the wing to swing down from the wing roots, giving the NA-73X a wide track of 3.66 meters (12 feet) that made it easy to handle on the ground. The tailwheel was steerable and retractable, sealed off in flight by two tiny doors. Aerodynamic controls were hydraulic, while armament controls were electric. The new aircraft had a sizeable fuel capacity for a fighter, a total of 681 liters (180 US gallons), almost twice the capacity of a Spitfire. There was one fuel tank in each wing, nested between the two wing spars against the wing root.
* The unarmed NA-73X was rolled out of the factory on 9 September 1940, 102 days after the contract was signed. The design team, which had been working all day, all week, for that time, collectively collapsed in exhaustion and took a few days off. They had the time to rest, since the prototype didn't have an engine; the engine was to be supplied by the USAAC, and naturally the service's own Allison-powered aircraft, such as the P-38, P-39, and particularly the P-40, had priority. The engine didn't arrive for about a month, and was installed in haste when it was finally delivered.
First flight of the NA-73X was on 26 October 1940, with NAA test pilot Vance Breese at the controls, and demonstrated the excellence of the design, though minor modifications were incorporated during the test flights. By this time, the British had ordered 320 of the fighters from NAA, and would quickly increase the order to a total of 620.
Unfortunately, the program ran into a serious delay. On 20 November 1940, NAA test pilot Paul Balfour was bringing the NA-73X back in toward the runway after the fifth test flight. He throttled back to lose airspeed, and the engine stalled and went dead. Balfour performed a dead-stick landing into a cultivated field just short of the runway, and the aircraft flipped onto its back in the soft ground. Fortunately the NA-73X didn't catch fire, since Balfour was trapped in the cockpit and had to be dug out. After he got out, he walked off, in bad humor but otherwise unharmed. The aircraft, in contrast, was seriously damaged, and wouldn't fly again until January 1941.
The problem was traced to the position of the entrance of the carburetor tube on the cowling above the engine. At low throttle and with the aircraft at a high angle of attack, airflow through the tube could be cut off, stalling the engine. The tube was extended to just behind the propeller, as mentioned earlier. Other changes from the initial prototype flight configuration included a modified belly scoop scheme, with the inlet moved forward a short distance, and replacement of the original one-piece "blown" windscreen to a framed windscreen.
By the time the prototype was in the air again, the type had acquired a name. In December 1940, the RAF named it the "Mustang", after a popular tune that had been a hit in America and Europe in the 1930s. The name stuck.
BACK_TO_TOP* The second Mustang to be built was the first production aircraft; it was rolled out on 1 May 1941, well behind schedule. The third Mustang, and second production item, was transported by ship to Liverpool, to make its first flight in Britain on 24 October 1941. It was fitted with British VHF radio, gunsight, and other incidental gear at the Aeroplane & Armament Experimental Establishment at Boscombe Down, and run through an initial evaluation. On 28 January 1942, the aircraft arrived at the Air Fighting Development Unit (AFDU) at RAF Duxford to begin a complete evaluation. Most of the first 20 Mustangs to arrive in England were used for test and evaluation.
RAF test pilots were delighted with the aircraft. In contrast with the Spitfire V, the Mustang was substantially faster (with a maximum speed of 615 KPH / 382 MPH) at low altitude; handled better in many respects; and most significantly, had over twice the endurance and range (over 1,600 kilometers / 1,000 miles). There were points of concern, such as the fact that the cockpit layout differed from that of other RAF fighters; in addition, the 7.62-millimeter (0.30-caliber) machine guns fitted to the Mustang used ammunition incompatible with the standard British 7.9-millimeter (0.303-caliber) guns.
The biggest concern, however, was the engine. The Allison V-1710-39 (also known as -F3R) engine provided 860 kW (1,150 HP), compared with 885 kW (1,185 HP) for the Merlin 45 used in the Spitfire V. The difference in power was slight, but the problem was that the Mustang weighed a third more than the Spitfire V, and so the Mustang was underpowered. In addition, although the Allison did incorporate a gear-driven supercharger, it was optimized for low-altitude operation. Development of the V-1710 had been guided by the requirements of the USAAC, and their doctrine had not emphasized high-altitude combat. The result was that the Mustang's performance fell off drastically above about 4 kilometers (13,000 feet).
For this reason the new aircraft, designated "Mustang I", was rejected by RAF Fighter Command, and assigned to the Army Co-Operation Command to act in the armed or "tactical" reconnaissance role. The Mustang was to replace the unpopular Curtiss (P-40) Tomahawks, which were often grounded by mechanical problems and which were no match for the Luftwaffe's fighters.
BACK_TO_TOP* By the end of 1941, the British had received 32 Mustangs, with more on the way. The Mustang Is began to be delivered to RAF front-line squadrons in January 1942, and entered operational service in February. These initial Mustangs were fully armed but had a reconnaissance camera, fitted in the left cockpit rear window, staring downward at an angle. Later, another camera was fitted just forward of the tailwheel. The RAF was concerned enough over the Mustang's general resemblance to the Messerschmitt Bf 109 to give it special recognition markings, consisting of yellow bands around the wings.
The RAF found that reconnaissance missions could also be combined with more aggressive actions, and that the Mustang I was up to both jobs. It was an accurate bomber, and could take on a Messerschmitt Bf 109 or Focke-Wulf Fw 190 on fair terms at low altitude. On 10 May 1942, an RAF Mustang I drew its first blood on a reconnaissance mission over the French coast when it shot up an airfield and a supply train. This action was quickly followed by enthusiastic RAF "reconnaissance and destroy" missions, codenamed "Poplars", over France. Such missions involved two Mustangs, one to take pictures and the other to keep an eye out for enemy fighters.
The first Mustang to be lost in action went down over France in July 1942. The next month, on 19 August 1942, the Mustang achieved its first air-to-air "kill", during the badly bungled British "practice invasion" at the port of Dieppe in France. The pilot of the Mustang was, by another of the small ironies of the story, an American, Hollis H. Hills, who was a volunteer flier with the Royal Canadian Air Force (RCAF). Hills would later join the US Navy and fly the Grumman F6F Hellcat. The Dieppe operation was otherwise an expensive one for the Mustang, however, with nine shot down and two more written off after their return to base.
By that time, the RAF was finding more uses for the Mustang, in the form of "Rhubarbs" and "Rangers". Both involved sending out pairs of Mustangs to attack German targets in occupied France and the Low Countries. In a Rhubarb, the aircraft were assigned to hit designated targets, while in a Ranger they picked targets of opportunity -- though the permissible list of targets was limited, in order to reduce injury to the local populations. The Mustang was also assigned to RAF Coastal Command to support patrol aircraft, as well as intercept low-flying Fw 190s making sneak raids on channel ports.
In October 1942, the long-range Mustang became the first RAF single-engine fighter to fly over Germany; restrictions on targeting were relaxed for attacks on German soil. By then, the initial batch of 320 NA-73 Mustang Is had been delivered by NAA, as well as most of the second batch of 300. 14 RAF squadrons were equipped with Allison Mustangs. The aircraft in the second batch were designated "NA-83", but incorporated only minor modifications and were hard to tell apart from the first batch. Most of the changes were added to allow the aircraft to operate more effectively under tropical and desert conditions.
* The Mustang was developed as a collaboration between NAA and the British, with little direct involvement by the USAAC. However, NAA did have to get US government approval to build combat aircraft for a foreign power, and as part of this "Foreign Release Agreement", the company was required to give two NA-73s to the USAAC. NAA received no payment for these two aircraft, but the possibilities of sales to the USAAC made it worthwhile.
The two aircraft, designated "XP-51", were the fourth and tenth machines off the production line. The first XP-51 was flown in late May 1941, but didn't make it to the USAAF (the "Air Corps" having been superseded by the "Air Forces" in June 1941) testing grounds at Wright Field, Ohio, until late August. The second XP-51 didn't get there until mid-December, by which time tests had been largely completed.
Although the evaluation was very positive, the USAAF was slow to take an interest in the Mustang. The service was, after all, preoccupied with the development of aircraft built to the USAAF's own specifications, such as the Lockheed P-38 Lightning. The Mustang was an outsider in their plans.
Making decisions in large bureaucracies is necessarily slow, since there are many people involved. Military organizations have particular quirks in this regard: they tend to move slowly in peacetime, and then go into a mad scramble when war comes along. A small, dedicated organization like North American found this sluggishness both hard to understand and frustrating. There are stories that there was a faction in the USAAF that was strongly opposed to the Mustang -- and even that one government official demanded a bribe from Dutch Kindelberger to allow the Mustang to be considered.
These stories sound suspiciously like exaggerations, and certainly would be difficult to verify. While a conspiracy of sorts might have existed, there is no need to invoke one to explain events. The USAAF's disinterest was perfectly logical under the circumstances, though later the commanding general of the USAAF, Henry H. "Hap" Arnold, was to say that it had been an extraordinary mistake that the Mustang was not put into service with the USAAF immediately.
BACK_TO_TOP* However, even before the USAAF received the two evaluation Mustangs, they had purchased 150 of them, sort of. In March 1941, US President Franklin D. Roosevelt had signed the "Lend-Lease Bill", which was a thinly concealed military assistance program for nations fighting the Axis powers.
The British had run low on cash, and so on 7 July 1941, the USAAC had ordered 150 "NA-91s" for them, which were given the formal designation of "P-51". This was before the RAF had even received its first evaluation Mustang. This Lend-Lease batch of aircraft had an unusual weapons fit, with gun armament consisting of four Hispano-Suiza Mark II 20-millimeter cannon, replacing the six wing-mounted machine guns.
The cannon armament was introduced at the request of the British, who had been plagued during the Battle of Britain with the feeble firepower of Hurricanes and Spitfires armed solely with banks of rifle-caliber 7.9-millimeter machine guns. The Mustang's two nose machine guns were deleted, with the three machine guns in each wing were replaced by two cannon. The Hispano Mark II had been designed as a "motorcannon", fitting over the cylinder banks of an inline vee engine and with the muzzle sticking out the prop spinner. That meant the weapon was very long and its muzzles distinctively protruded well out in front of the wing of the P-51. Despite the hitting power of this heavy armament, for whatever reasons no other major model of Mustang would feature it.
Deliveries began in July 1942, with the RAF giving the new version the designation of "Mustang IA". However, the US Navy diverted one of them for evaluation, while the USAAF diverted 57 more for its own use. Two of these were set aside for engine experiments that would prove significant and which are discussed later.
The 55 remaining P-51s were fitted with two reconnaissance cameras in the rear fuselage; they retained the four cannon, being accordingly labeled tactical reconnaissance machines. They were supposedly redesignated "F-6As", but in practice continued to be known as P-51s, or sometimes "P-51-1s". The USAAF also referred to early Mustangs under the name "Apache", but this usage was very short-lived, since the British name quickly triumphed for all P-51 variants.
BACK_TO_TOP* By this time, the USAAF was warming to the Mustang, but there was a problem: the US military air fleet was expanding so fast there wasn't any money to buy more fighters for the moment. There was money for attack aircraft, though, and after discussions with the USAAF, NAA came up with a Mustang tailored for the attack role.
Dive bombers were sexy at the times, and so NAA offered the USAAF a dive-bomber version of the Mustang, designated "NA-97". On 16 April 1942, the USAAF ordered 500 of these aircraft, formally designated "A-36A Invader", though the type would still be referred to in practice as a Mustang. These were to be the first Mustangs to be specifically ordered for American service. The A-36A first flew on 21 September 1942, with NAA test pilot Ben Chilton at the controls. The A-26 was powered by an uprated Allison V-1710-87 engine, with 990 kW (1,325 HP) at low altitude. The A-36A was stressed for high-speed dives, and included fence-style hydraulically-operated dive brakes on the top and bottom of each wing.
The previous armament fit of two 7.62-millimeter guns and one 12.7-millimeter gun in each wing was changed to two 12.7-millimeter Browning guns per wing. 7.62-millimeter guns were no longer perceived as packing enough punch for air combat, and stocking two different calibers of ammunition for an aircraft was a logistical nuisance. Reducing the number of machine guns in each wing from three to two also simplified the cramped and cluttered gun arrangement inside the wing.
The two 12.7-millimeter Brownings in the nose were retained, resulting in a total gun armament of six 12.7-millimeter guns -- though apparently the nose guns were often deleted in the field to save weight. There was a stores pylon under each wing for a 225-kilogram (500-pound) bomb, smoke generator, or 284-liter (75 US gallon) drop tanks. A single A-36A would be passed on to the British in March 1943; this one aircraft was designated in RAF service as the "Mustang I (Dive Bomber)".
* The USAAF soon received more money to buy fighters. The ploy of buying them as a dive bomber no longer being needed, the USAAF placed an order for 310 "NA-99" fighters in August 1942. These aircraft, which were designated as "P-51A" by the USAAF, were very similar to the A-36A, but the dive brakes were deleted, as were the nose guns with their heavy synchronization gear. They also featured an Allison V-1710-81 engine with a new supercharger for somewhat better high-altitude performance, and a larger propeller.
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NORTH AMERICAN P-51A (MUSTANG II):
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wingspan:
11.28 meters (37 feet)
wing area:
21.65 sq_meters (233 sq_feet)
length:
9.83 meters (32 feet 3 inches)
height:
3.71 meters (12 feet 2 inches)
empty weight:
3,000 kilograms (6,600 pounds)
max loaded weight:
4,000 kilograms (8,800 pounds)
maximum speed:
628 KPH (390 MPH / 340 KT)
service ceiling:
9,100 meters (30,000 feet)
range with drop tanks:
2,000 KM (1,250 MI / 1,085 NMI)
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The first P-51A took to the air on 3 February 1943, piloted by Ben Chilton, and initial production deliveries followed the next month. 35 of these aircraft were fitted with two cameras and designated "F-6Bs" -- though it seems that designation wasn't actually applied until late in the war -- while 50 of them were passed on to the British as "Mustang IIs". These were the last Allison Mustangs built.
By this time, the USAAF had realized the potential of the Mustang. America became the chief patron of an aircraft that had been designed for the British, and which had been ignored by the USA for what some felt was far too long a time.
BACK_TO_TOP* Although the RAF had been using Mustangs in combat since the spring of 1942, the USAAF didn't get the type into action until March 1943, after the invasion of North Africa. 35 of the "F-6A" P-51s arrived in that month, and were used in the following months for tactical reconnaissance on German positions in Tunisia during the final drive on the Afrika Korps.
In the meantime, A-36As were pouring into North Africa by the hundreds, with a total of over 300 in action by May. On 7 June, the USAAF performed its first combat mission with the Mustang, performing attacks on the Mediterranean island of Pantelleria. The A-36As participated in the invasion of Sicily in July 1943, and then, having been joined by P-51As, moved to that island to support the landings on the Italian mainland at Salerno in September 1943.
The combat record of the A-36A as a dive bomber is obscure: some sources claim that the aircraft was much too "slick" and fast to be a good dive bomber, and in fact there were a few fatal accidents during stateside training from structural failures in dive attacks. There were also reports that the dive brakes didn't deploy evenly, making the aircraft more difficult to control in a dive. Stories that the dive brakes were often wired shut, however, appear to be an exaggeration, possibly derived from the fact that they were sometimes wired shut on old and weary aircraft, since it was less trouble than fixing them. Problems with the dive brakes were corrected in any case, and some USAAF reports indicate that the A-36A was a highly accurate dive bomber.
The matter is mostly academic, because it is entirely clear that the A-36A, as well as the P-51A, gave fine service as an attack aircraft during the Sicilian and Italian campaigns. The Mustangs worked hard during the Salerno landings, flying off beachhead air strips while under enemy fire. Combat operations unfortunately highlighted the vulnerability of the Mustang's liquid-cooled engine to ground fire, as well as a high rate of "friendly fire" casualties due to its resemblance to the Bf 109. By March 1944, roughly half of the Allison Mustangs operated by the USAAF in Italy would be gone due to various causes, and the rest would be replaced by July 1944.
The Allison Mustangs contributed a great deal to the Allied war effort as close-support aircraft. One pilot, USAAF Lieutenant Michael T. Russo, was skillful and lucky enough at air combat to become the sole Allison Mustang ace of the war.
* In the meantime, the A-36A and P-51A were demonstrating similar virtues at the other end of the Eurasian land mass. A USAAF Mustang fighter-bomber bomber group arrived in India by way of Australia in the late summer of 1943, where it performed attack, reconnaissance, and air combat duties in the China-Burma-India (CBI) theatre.
The Allison Mustangs did not do well in the air combat role. They were at a disadvantage against their primary adversary, the agile Nakajima Ki-43 Hayabusa, known to the Allies as the "Oscar". Although the Oscar had significant weaknesses, being under-armed and lacking in armor protection, the Mustangs got the worst of it in their clashes with the Japanese fighter over Burma. These missions were, however, the first use of the Mustang as a long-range escort fighter, with P-51As carrying two 284-liter (75 US gallon) drop tanks and flying round trips from Kurmitola in Bengal to Rangoon in Burma, 725 kilometers (450 miles) away. It was a hint of things to come.
The Mustangs did well at reconnaissance and close support missions. One P-51A outfit, the 1st Air Commando Group, was assigned to provide close support for Orde Wingate's British Chindit warriors in Burma, and operated in the field at rough airstrips to keep up with the battle. They sometimes carried 450-kilogram (1,000-pound) bombs, or the three-tube 11.5-centimeter (4.5-inch) M-8 "bazooka"-style rocket launchers.
The 1st Air Commando group was led by Lieutenant Colonel Philip Cochran, a legendary old China hand who was well-known to the American public through the cartoon character "Flip Corkin" in Milton Caniff's TERRY & THE PIRATES comic strip. Although details are sketchy, the Allison Mustangs appear to have operated in the CBI well into 1944, when they were replaced by P-51Bs.
* While USAAF pilots were cutting their teeth on the Allison Mustangs, the RAF continued to use them to good effect for reconnaissance and attack. The RAF Army Co-operation Command was disbanded in June 1943, with the Allison Mustangs merging back into RAF Fighter Command for a time, and then sent to 2nd Tactical Air Force, which was put together to support the invasion of Europe.
Attrition and war-weariness reduced the numbers of Allison Mustangs in Britain as they were replaced by better types, but on D-Day there were three RAF, two RCAF, and one USAAF squadrons operating them from the UK. Their numbers continued to dwindle, but by the war's end, the RAF was still operating two squadrons of them, and the USAAF was performing tactical reconnaissance with the handful remaining. The RAF finally phased theirs out at the end of 1945, with about 72 of them on the lists at the time.
BACK_TO_TOP* A total of about 1,583 Allison-powered Mustangs was built. Even if the aircraft's evolution had stopped there, these Mustangs were useful enough to give distinction to the type. After the war, the USAAF compiled statistics on the A-36A variant and concluded that the A-36As had flown over 23,000 sorties and destroyed 86 enemy aircraft in air combat, despite being a "mudfighter". The other Allison Mustang variants were no doubt put to similarly good use.
There were a number of experimental modifications of the Allison Mustangs, especially in British service.
In December of 1942, the British modified a Mustang I to carry eight 7.62-centimeter (3-inch) "rocket projectiles (RPs)", but the launch rails added enough drag to cut the top speed of the aircraft by 110 KPH (70 MPH). The British tried again in the spring of 1943 with improved launch rails, but with no better effect. The Americans would qualify the A-36A and P-51A for use with the M-8 bazooka-tube launcher, as mentioned, though this caused performance problems as well.
The British performed particularly interesting experiments, beginning in mid-1943, with a Mustang I fitted with a modified wing and tested with various underwing stores, the most spectacular being two Vickers 40-millimeter "S" guns, one under each wing. The two 12.7-millimeter guns in the nose were retained in this configuration, but the wing armament was reduced to a single 7.62-millimeter gun in each wing. This particular weapons fit was not judged worth pursuing further, and no other Mustang ever featured it. The same could be said of one Mustang I that the British fitted with swiveling landing gear to improve takeoff and landings in crosswinds, and of two P-51As that the USAAF sent to the Aleutians and tested with ski landing gear.
* Relatively few Allison Mustangs have survived to the present day. The well-known Chino "Planes of Fame" museum operated a P-51A in air races, and two A-36As were said to have flown the Reno races as well. One particularly prominent example of the types that has survived is the initial XP-51 obtained by the USAAC for testing in the summer of 1941. This aircraft was scheduled for scrapping after the war, but was rescued for the Smithsonian's National Air & Space Museum. In the 1970s it was restored and flew again, and is now in the hands of the Experimental Aircraft Association in Oshkosh, Wisconsin.
Incidentally, modelers have liked to play with Mustangs fitted with radial engines, generally known by the "what-if" designation of "P-51R". It doesn't appear anyone ever tried to fit a radial into a P-51 in practice, and it poses two questions: could a radial of adequate power be compact enough to be reasonably fitted to a Mustang? Even if it could, would it still ruin the slender P-51's aerodynamics?
There is a hint that it could be done. The Japanese built a fighter during the war, the Kawasaki "Ki-61 Hien", that had a vee-12 engine and a configuration much along the lines of the Mustang. Late in the war, in response to a shortage of vee-12 engines, Ki-61s were fitted with radial engines, with the "Ki-100" proving to have excellent performance.
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