Mustang I and IA for the RAF

by Joe Baugher

The P-51 Mustang was perhaps the most famous fighter of World War II, and, many would say, the best all-round piston-engined fighter produced by any of the combatants during that conflict. Total production of all Mustangs amounted to 15,575 in the USA and 100 in Australia, ranking only behind the P-47 Thunderbolt in being the fighter manufactured in greatest numbers for the USAAF. Mustangs accounted for 4950 of the 10,720 air combat victories claimed by the USAAF in Europe, and 4131 of the 8160 ground strafing claims made in the same theatre, accounting for 48.9 percent of total losses inflicted on the enemy. They shot down more than 230 V-1 "buzz bombs", and they even managed to score some kills against Luftwaffe jet fighters.

In 1934, James H. "Dutch" Kindelberger became president of North American Aviation, Inc. Not only was Kindelberger an excellent businessman, he was also a capable aeronautical engineer. As early as 1938, Kindelberger had made numerous trips to Europe seeking orders for his company, and he had the opportunity to see up close some of the airplanes that would be in combat in the war that almost everyone believed would shortly be coming. After hostilities broke out, Kindelberger eagerly sought out combat reports from both sides and developed some ideas of his own. Although Kindelberger had no experience with fighters, he collaborated with his friend and colleague J. Leland Atwood to formulate an outline for a fighter project. A project team was formed at North American, made up of such people as Raymond H. Rice, Edgar Schmued, Larry Waite and E. H. Horkey. A sort of urban legend has grown up about Edgar Schmued, which claims that he had once worked for Willy Messerschmitt and that the Mustang was heavily influenced by the Bf 109.

In April of 1940, Kindelberger was summoned by the British Air Purchasing Commision and asked to manufacture the Curtiss Hawk 87 (P-40D) under license for the RAF. Kindelberger countered that NAA could do better than that airplane and that they could design a real fighter in the same time that it would take to put the P-40 into production. The British commission felt that they could take Kindelberger at his word, and on April 10, 1940 they accepted his proposal on the condition that the first prototype be ready in 120 days. The design was assigned the company project name of Model NA-73.


At that time, the USAAC reserved for itself the right to block any foreign aircraft sales that it regarded as not in the Army's interest, for whatever reason. On May 4, 1940, the US Army reluctantly agreed not to block the British sale, but they added a condition. Two examples of the initial lot for Britain were to be transferred to the USAAC for testing free of charge.

The NA-73X prototype contract was signed on May 23, 1940. The British insisted that a heavy eight-gun armament be fitted. NAA had actually been quietly working on such a fighter project since the summer of 1939, and by that date they had actually already completed much of the detail design. On May 29, a provisional RAF procurement was issued for 320 aircraft, contingent on satisfactory testing of the prototype. NAA agreed to start deliveries in January 1941. RAF serial numbers were to be AG345 through AG664, and the aircraft was given the name Mustang I in RAF service.

Another urban legend surrounding the Mustang is that it owed a great deal to the Curtiss P-40, and, in fact, stole numerous design features from that fighter. Although NAA did pay $56,000 for technical aerodynamic data on the Curtiss XP-46, the NA-73X owed virtually nothing to any previous design and certainly did not owe anything to the Curtiss P-40.

The NA-73 featured an all-metal stressed-ski structure, with the wing having a sheet-web main spar and an almost equally strong rear spar to carry the ailerons and the flaps. Special attention was paid to features which would make the aircraft simple and inexpensive to manufacture. The two wing spars had to be far enough apart to accommodate the length of a 0.50-in machine gun, with only the barrel protruding ahead of the main spar. Most previous NAA aircraft had left and right wings bolted to a horizontal center section, but the Mustang had the wings meeting on the centerline, with dihedral emanating from that line.

A special NACA laminar flow wing profile was adopted for the Mustang. This was an aerofoil which had a thickness that kept on increasing far beyond the usual location, i.e., to 50 percent chord rather than the usual 20 percent. These profiles had little camber, the undersurface being almost a mirror image of the upper. This wing was much more "slippery" than the old profiles, and provided lesser aerodynamic drag at high speeds than did more conventional aerofoils. However, it also had less lift at low speeds, so the NA-73X had to have large and powerful flaps to keep landing speeds from being impractically high.

The British also specified that a liquid-cooled inline engine be used, and the Allison V-1710 twelve-cylinder Vee was the only American-built engine which fit the bill. The Allison V-1710 was a little bigger than the Merlin, slightly lighter, and similar in power at low altitudes. However, at higher altitudes the Allison suffered from a rapid drop in power in comparison to the Merlin. NAA briefly considered using a supercharger to improve high-altitude performance, but ruled against it on the grounds of a tight schedule.

The Allison engine had a downdraft carburetor, so the ram inlet of the NA-73X was located above the cowling. Radiators for cooling the ethlyene glycol and lubricating oil were located in a single heat-exchanger installed underneath the rear fuselage in a streamlined duct. The drawback of such an arrangement was the extra weight and combat vulnerability of the long pipes that led to and from the engine. Military innovations such as self-sealing fuel tanks, cockpit armor, and a bulletproof windshield were to be provided from the start.

In a contract approved on September 20, 1940, it was agreed that the fourth and tenth production NA-73s would be the planes diverted to the Army. The designation XP-51 was to be assigned to these two planes.

On September 24, 1940, the RAF increased their Mustang I order to 620 planes.

The NA-73X prototype emerged from Inglewood plant in only 102 days, thus meeting the 120-day deadline with time to spare, although the airplane rolled out of the factory without an engine, which had been delayed at the Allison factory. The engine that was eventually installed was an unsupercharged Allison V-1710-F3R liquid-cooled Vee, rated at 1100 hp. The NA-73X bore the civil registration NX19998. Veteran test pilot Vance Breese flew it for the first time on October 26, 1940. Weights were 6278 lbs empty, 7965 lbs normal loaded. It was a clear 25 mph faster than the P-40, even in spite of being powered by the same engine.

Following tests, there were several changes in the geometry of the ventral ducting and the controllable flaps. By the time that the NA-73 had been cleared for production, the duct had had its inlet moved downward so that its upper lip was lower than the underside of the wing, thus avoiding the ingestion of a turbulent boundary layer.

On November 30, 1940, while on the third test flight of the NA-73X, another test pilot forgot to change fuel tanks, ran out of gas, and suffered a forced landing in a farmer's field. This mishap put the prototype out of action for several months. However, since this accident was not the fault of the aircraft itself, this did not unduly delay the program.

Since the NA-73X had encountered very few problems during tests, production for the RAF began almost immediately. The first production Mustang I for the RAF (AG345) flew for the first time on April 23, 1941, well behind the original schedule. It was retained by NAA as a development machine, and was used in an extensive series of tests to iron out bugs and eliminate problems. Perhaps the most noticeable change was the extension of the carburetor inlet right up to the nose in order to give good ram recovery at extended angles of attack. This machine was initially unpainted, but it later got an RAF paint job with camouflage, but it remained at Inglewood and did not ever get any guns.

Armament was fitted to the second aircraft off the production line (AG346). It was equipped with four 0.50-in machine guns and four 0.30-inch guns. Two of the 0.50-in guns were mounted in the lower fuselage and were synchronized to fire through the propeller arc. The rest of the guns were mounted in the wings and fired clear of the propeller arc. This aircraft was accepted by the RAF in September and started a long journey to Britain, finally arriving in Liverpool on October 24, 1941. It lacked a radio, a gunsight, and certain other equipment which was by contract to be supplied by British manufacturers. Once the British equipment was installed, the complete aircraft was evaluated at the Aeroplane & Armament Experimental Establishment at Boscome Down and by the Air Fighting Development Unit at RAF Duxford.

The newly-arrived Mustang was quickly recognized as being the best fighter aircraft yet to be delivered from the USA. It was found to be superior to the Kittyhawk, Airacobra and Spitfire in both speed and maneuverability at low altitudes. Maximum speed was 382 mph at 13,000 feet. The armament of four 0.50-inch and four 0.30-inch machine guns was heavy and effective. Handling was excellent. The range was nearly double that of any RAF single-engined fighter. It was 25 to 45 mph faster than the Spitfire V at altitudes up to 15,000 feet. The problem was the rapid fall-off in performance at altitudes above 15,000 feet, caused by its low-altitude Allison engine. This was more than just a minor deficiency, since most aerial combat over Europe at that time was taking place at medium to high altitudes. Consequently, it was decided that the Mustang I could be best used for low-level tactical reconnaissance and ground attack, where full advantage could be taken of its exceptional low-altitude performance.

Most of the first 20 RAF Mustang Is were retained for special measurements and trial installations. Mustangs delivered under the original contract were similar to the original model but had an F-24 camera mounted in an installation immediately behind the pilot's head armor, looking obliquely out to the left and to the rear. A single gun camera was added near the left wing tip.

It was initially feared that the Mustang I might be mistaken for a Bf 109 during the stress of combat, and most of the Mustang Is in front-line RAF service had bright yellow bands painted across their wings.

The first RAF unit to receive the Mustang was No 26 Squadron at Gatwick which began to operate the fighter in February 1942. In April, two more squadrons received Mustangs, and eight more in June. Most of the aircraft went to Army Cooperation Command, usually replacing Curtiss Tomahawks or Westland Lysanders. The first Mustang combat mission was undertaken by Flying Officer G. N. Dawson of No. 26 Squadron on May 10, 1942, strafing hangars in France and shooting up a train.

The first Mustang I operational sortie was on July 27, 1942. Mustang Is participated in the disastrous Dieppe landings by British commandos on August 19, 1942, where it saw the first air-to-air action. During this operation, pilots of No 414 Squadron of the RCAF were attacked by Fw 190s. An American RCAF volunteer, F/O H. H. Hills, shot down one of the enemy, which was first blood for the Mustang.

In October of 1942, On a mission to the Dortmund-Elms Canal and other objectives in Holland, the Mustang I became the first single- engined fighter based in the UK to penetrate the German border. By this time, the Mustang I equipped Nos 2, 4, 16, 26, 63, 169,239, 241, 268, and 613 Squadrons of the RAF, plus Nos 400, 414 and 430 Squadrons of the RCAF, and No 309 (Polish) Squadron of the RAF.

In December 1940, the RAF ordered 300 more of the Mustang Is which embodied only minor modifications. These were designated NA-83 by the factory. RAF serials were AL958/AL999, AM100/AM257, and AP164/AP263.

Mustang I AM106 was experimentally fitted successively with eight rocket projectiles on zero-length launches, special long-range fuel tanks, and eventually with two 40-mm Vickers cannon in underwing mountings.

With the passage of Lend-Lease, 150 more Mustangs were ordered by the US Army, all of which were intended for delivery to Britain. These were designated Mustang Mark IA by the RAF and NA-91 by the factory. The Mustang IA differed from earlier versions in having the machine guns replaced by four 20-mm wing-mounted Hispano cannon, with most of the long barrels protruding well ahead of the wing. For contractual purposes, these aircraft were given the US designation of P-51 and assigned the USAAF serials 41-37320/37469. The RAF serial numbers assigned to this lot were FD418/FD567. The Allison V-1710-F3R engine was given the US Army designation V-1710-39. Throughout 1941, the Army referred to these aircraft under the name Apache, but this was changed to Mustang at about the time the deliveries began in mid-1942.

The British did not get all of these NA-91s. Since the RAF deliveries took place after Pearl Harbor, many were repossessed by the Army before they reached England. These included RAF Mustang IA serials FD418/FD437, FD450/FD464, FD466/FD469, and FD510/FD527. The Army planes were armed with four 0.50-inch machine guns rather than the 20-mm cannon and were fitted with two K-24 cameras in the fuselage. Most retained their RAF camouflage and serial numbers, although some were indeed painted with their equivalent USAAF serials. These were designated as tactical reconnaissance aircraft and were designated F-6A, but this designation was soon changed to P-51.

Tactical reports from RAF army cooperation units were laudatory. The Mustang I and IAs were able to take an incredible amount of battle damage. The long range of the Mustang made it an excellent tactical reconnaissance aircraft and its heavy armament made it effective against most ground targets. In 18 months of operation 200 locomotives and 200 barges were destroyed or severely damaged, and an undetermined number of enemy aircraft were destroyed on the ground. This was accomplished at the expense of only one Mustang being shot down by enemy fighters, five lost to flak, and two vanishing with no record of their fate. At low altitudes, the Mustang was faster than either the Bf 109 or the Fw 190. At sea level, the Mustang could run away from any enemy aircraft. The flaps were very useful in combat to reduce the turning radius.

Mustang Is and IAs served with the RAF up until 1944. It knew few equals in the role of low-altitude interdiction and reconnaissance.

In March of 1943, a batch of 35 P-51/F-6As were assigned to the 154th Observation Squadron at Oujda in French Morocco. This was the first US Mustang unit. Its first mission was a photographic coverage of Kairouan airfield, Tunisia on April 10, 1943.