Modeller's Guide to Early P-51 Mustang Variants
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.
This article covers the initial period of Mustang development, namely the Allison-powered variants. There has been much confusion in designation of different variants during this period, I hope to straighten things out for those unaware of all the details.
The Mustang Origin
Following the outbreak of war in Europe, the British Purchasing Commission, headed by Sir Henry Self, was posted to New York to determine if American combat aircraft could be of any use to the Royal Air Force. The Bell P-39 Airacobra and the Curtiss P-40 Warhawk were ordered in substantial numbers, even though they were not up to the performance standards of the latest British and German fighters.
One of the corporations that Self had contacted had been the North American Aviation corporation. North American had already been building NA-16 trainers, and the British ordered a number of them for the RAF as the Harvard. In April of 1940, the manager of NAA James H. "Dutch" 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, who was an excellent businessman as well as aeronautical engineer, responded that NAA could do that if it were really required, but countered that he and his company could build a better fighter than the P-40 and that they could design a real fighter in the same time that it would take to put the P-40 into production. 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 as early as summer 1939. 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.
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 NA-73 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.
Another urban legend surrounding the Mustang is that it owed a great deal to the Curtiss XP-46 and, in fact, stole numerous design features from that fighter. It is true that the British had insisted that since NAA had no fighter experience they should secure all current data from Curtiss about both the P-40 and the XP-46. Although NAA did pay $56,000 to Curtiss for technical aerodynamic data on the XP-46, there was only a very broad resemblance between the XP-46 and the NA-73X. The Curtiss aircraft shared only a similar radiator/ oil-cooler configuration with the NA-73X, and did not have laminar flow wings. In point of fact, the development of the XP-46 lagged behind that of the NA-73X, and prototypes were not ready for flight until February of 1941. In addition, preliminary design of the NA-73X was completed before NAA gained access to the Curtiss material. It could even be argued that the XP-46 data was most useful to NAA in guiding them in what NOT to do. The NA-73X appears to owe virtually nothing to any previous fighter design. Nevertheless, despite convincing denials from both Edgar Schmued and aerodynamicist Edward Horkey, the full magnitude of the contribution of Curtiss to the NA-73X design remains controversial to this day.
The NA-73 featured a carefully streamlined all-metal stressed-skin structure, revolutionary laminar-flow wing, powerful flaps, wide-track landing gear and fully retractable tail wheel. 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. Fuel tanks of large capacity were placed in the wings.
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. The leading and trailing edges were straight lines to the extent possible, and the underlying structure was simple and easy to manufacture.
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 turbosupercharger to improve high-altitude performance, but ruled against it on the grounds of a tight schedule.
At British insistence, armament was to be somewhat heavier than American standards of the day. Two 0.5-inch M2 Browning machine guns were installed in the underside of the nose beside the engine crankcase, synchronized to fire through the propeller arc. The left gun was staggered ahead of the right in order that the magazines could lie one behind the other. Two 0.50-inch guns were mounted upright inside the wings, outboard of the landing gears. Four 0.30-inch Browning machine guns were mounted further outboard on the wing, with each inboard 0.30-inch gun being mounted lower so that its muzzle was below the leading edge.
Final assembly and engine installation began on September 9, 1940, 102 days after the initial British order, 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. In the absence of the new disk brakes, the aircraft was rolled on wheels borrowed from an AT-6 trainer. It was completely unpainted except for six aperture shapes painted on the wing leading edges to show where the guns would be installed. These aperture shapes were retouched out in many reproductions of the most famous photographs of the aircraft. Only later was the civil registration NX19998 applied and the fuselage ahead of the cockpit painted with anti-dazzle black.
Veteran test pilot Vance Breese flew the NA-73X for the first time on October 26, 1940. 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 of air into the radiator cooler.
On November 20, 1940, while on the fifth test flight of the NA-73X, test pilot Paul Balfour forgot to change fuel tanks, ran out of gas, and suffered a forced landing. The plane ended up on its back 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. The NA-73X aircraft resumed flying on January 11, 1941 and continued in the initial development program until being retired on July 15, 1941.
Mustang Mk. I for the RAF
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.
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. Later, a second cameera was installed vertically ahead of the tailwheel for photography from higher altitudes.
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. They differed from the NA-73s only in having broad fishtail ejector exhausts.
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.
Mustang Mk. IA
On March 11, 1941, the Lend/Lease Act was passed by Congress, permitting the "lending" of American-built aircraft to nations deemed "vital to the security of the United States". On September 25, 1941, the US Army ordered 150 Mustangs under the provisions of Lend-Lease for delivery to Britain. All previous RAF Mustangs had been direct purchases by Britain.
These Lend-Lease Mustangs were designated Mustang Mark IA by the RAF and NA-91 by the factory. For contractual purposes, these aircraft were assigned the US designation of P-51 and were assigned US serial numbers. The Allison V-1710-F3R engine was given the US Army designation V-1710-39.
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. 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.
This factory-fresh NA-91 displays
Photo: American Memory Digital Library
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 and designated F-6A and P-51, of which more later.
The British decided that the relatively poor high altitude performance of the Mustang 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, the Mustang I was used for low-level tactical reconnaissance and ground attack, where full advantage could be taken of its exceptional low-altitude performance.
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. By October 1942 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.
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.
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 1940, the US Army had given its permission for the initial British Mustang delivery to proceed, with the proviso that two of the NA-73s destined for England be made available to the Army for tests free of charge. In a separate contract dated September 20, 1940, the two aircraft delivered to the Army were to be the fourth and tenth production NA-73s, and the planes were to be designated XP-51.
The fourth and tenth NA-73s were duly delivered to the US Army in May of 1941 for testing at Wright Field, Ohio. They were initially unpainted except for national insignia and the black antiglare panel over the forward fuselage ahead of the pilot. The Army painted the serials 1038 and 1039 on the fin and on each side of the nose, together with the WRIGHT arrowhead emblem on the rear fuselage. Much later, they were both painted olive drab overall.
Only after Pearl Harbor did the US Army finally agree to order the Mustang for its own use. On April 16, 1942, the Army ordered 500 NA-97s. The NA-97 was a ground attack version and was designated A-36A - in the attack series rather than the fighter series.
The A-36 seems to have been known by several different names--it was initially called Apache, which was the name that the Army initially assigned to the P-51, but there was an effort to change the name to Invader following the invasion of Sicily. However, the name Mustang was generally applied by most people to the A-36.
The A-36A differed from previous Mustang versions in having a set of hydraulically-operated perforated door-type dive brakes mounted at approximately mid-chord on both the upper and lower wing surfaces outboard of the wing guns. The brakes were normally recessed into the wings, but were opened to 90 degrees by a hydraulic jack to hold diving speeds down. A rack was fitted under each wing for a 500-pound bombs, a 75 US gallon drop tank, or smoke-curtain equipment.
A built-in armament of six 0.50-inch machine guns (two in lower fuselage nose, four in the wings) was fitted, however the two nose guns were often omitted in service. The wing guns were moved closer to the main landing gear strut in order to minimize stress under taxi and takeoff conditions. The engine was the Allison V-1710-87 (F21R).
The first A-36A flew on September 21, 1942. Deliveries of the A-36A were completed by the following March. The A-36A equipped the 27th and 86th Fighter Bomber Groups based in Sicily and in Italy. They initially were painted in olive-drab and light-gray finish and were painted with yellow wing bands and yellow circles around the national insignia. Both of these Groups arrived in North Africa in April of 1943 just after the end of the Tunisian campaign. The only other A-36 user was the 311th Fighter Bomber Group, based in India. It saw extensive use in the China-Burma-India theatre.
A sort of urban legend has sprung up about the A-36A's dive brakes. According to some stories, the dive brakes of the A-36A were next to useless and were deliberately wired shut at the manufacturers so that they could not be used. This story is totally incorrect. On the contrary, the dive brakes proved to be quite effective in combat, and the aircraft was so stable with the dive brakes extended that bombing while in a dive was particularly accurate. The origin of this legend seems to have been in the United States, at a time before the Invaders first went overseas. It seems that A-36A pilots were told by their officers in the USA that their dive brakes would be all but useless in combat and it would be best if they simply wired them shut. This turned out to be incorrect, and the dive brakes were used to great effect throughout the Sicilian campaign and the Italian invasion.
One A-36A was supplied to the RAF in March of 1943 for experimental purposes. Its RAF serial number was EW998.
The British did not get all of the NA-91 Mustang IAs produced. Since the RAF deliveries began after Pearl Harbor, many aircraft were repossessed by the Army before they reached England. 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.
British Mustang IAs to F-6A configuration. Note the blanked-off cannon stubs.
Photo: American Memory Digital Library
The P-51s went to Peterson Field in Colorado, where they were assigned to the newly-established aerial reconnaissance school. In March of 1943, a batch of 25 F-6A/P-51s were assigned to the 154th Observation Squadron at Oujda in French Morocco. This was the first US Mustang unit. The first mission was a photographic coverage of Kairouan airfield in Tunisia on April 10, 1943, which was the first USAAF Mustang mission of the war. No. 225 Squadron of the RAF frequently borrowed Mustangs from the 154th to augment its shorter- range Spitfires.
The next Army contract for Mustangs consisted of an order in August 1942 for 1200 NA-99 versions with the USAAF designation of P-51A. Unlike the A-36A, these aircraft from the start were meant to be fighters, not bombers. In the event, only 310 P-51As were actually built between March and May of 1943 before production was switched over to the Merlin-powered P-51B.
These aircraft had the same external stores capability as the A-36A Invader, but had no dive brakes and no fuselage guns, the armament being limited to four 0.50-inch machine guns mounted in the wings. The P-51A had the Allison V-1710-81 (F20R) engine with significantly better high-altitude performance than the V-1710-39 of the P-51. It also was fitted with a new supercharger which further enhanced low-altitude performance. In addition, a larger-diameter propeller was fitted.
The first P-51A group was the 54th, which remained in Florida for replacement training. Later, P-51As went to Asia with the 23rd, 311th, and 1st Air Commando Groups. Almost all of the P-51As served in the China, Burma, India (CBI) theatre of operations.
One P-51A was given to the US Navy for evaluation (BuAer #57987).
35 of the P-51A produced were fitted with twin-K24 camera installations and had their guns removed. These were redesignated F-6B. All aircraft served in Europe, mainly with the 107th Tactical Reconnaissance Squadron based in England.
Mustang Mk. II
50 P-51As went to the RAF, becoming Mustang IIs. These planes replaced the NA-91s that had been diverted from RAF Mustang IA orders for conversion as F-6As. Deliveries were made late in 1942.
The Mustang I, IA, and II had astonishingly long service with the RAF, with the last front-line RAF Allison-engined Mustangs being phased out in early 1945.
Mustang II FR901 was fitted with special deep-section fuel tanks beneath the wings for ultra-long-range flying.
On to the Merlin Engine
On April 30, 1942, Ronald W. Harker, a test pilot for the British Rolls-Royce engine manufacturer, took a brief hop in a RAF Mustang at the airbase at Duxford. Like lots of other pilots, he was highly impressed with the Mustang. It was 30 mph faster than the Spitfire VB at similar power settings and had nearly twice the range. Upon landing, he is reported to have said that the airplane would be a natural for the new Merlin 60 series of engines that Rolls Royce was just beginning to produce. Rolls Royce management immediately jumped into action, and a process was initialized which made the Mustang into the finest fighter of the World War 2. But that's a completely different story...
When production of the Allison-engined Mustang ended, 1580 examples had been built.
The table below states serial numbers for all Allison-powered P-51s produced.