Over twenty variants of the North American P-51 Mustang fighter were produced from 1940, when it first flew, to after World War II, some of which were employed also in the Korean War and in several other conflicts.
Allison-engined Mustangs
NA-73X
The prototype of the Mustang, designated NA-73X, was rolled out by North American Aviation on 9 September 1940, albeit without an engine, and was first flown the following 26 October.Kinzey 1996, p. 5. The Mustang was originally designed to use a low-altitude rated Allison V-1710 engine. Unlike later models, Allison-powered Mustangs were characterized by the carburetor air intake placed on the dorsal surface of the nose, immediately behind the propeller.
Mustang Mk I (NA-73 and NA-83)
The first production contract was awarded by the British for 320 NA-73 fighters, named Mustang Mk I by the British Purchasing Commission; a second British contract soon followed, which called for 300 more (NA-83) Mustang Mk I fighters. Contractual arrangements were also made for two aircraft from the first order to be delivered to the USAAC for evaluation; these two airframes, and respectively, were designated XP-51.Kinzey 1996, p. 12. The first RAF Mustang Mk Is were delivered to 26 Squadron at RAF Gatwick in February 1942 and made their combat debut on 10 May 1942. With their long range and excellent low-altitude performance, they were employed effectively for tactical reconnaissance and ground-attack duties over the English Channel, but were thought to be of limited value as fighters due to their poor performance above .
RAF Mustangs (Mk Is, which were not drop tank capable) made history on October 22, 1942, when they escorted 22 Vickers Wellington medium bombers on a daylight raid to Germany, thus becoming the first RAF single-seat fighters to fly over the country during World War II.Chorlton (2012), p. 39.
P-51/Mustang Mk IA (NA-91)
The first American order for 150 P-51s, designated NA-91 by North American, was placed by the US Army on 7 July 1940.Kinzey 1996, pp. 7, 17. This was on behalf of the RAF in a Lend-Lease deal.Chorlton, p. 22. All but the last 57 went to the British. After the attack on Pearl Harbor, the USAAF 'held back' these Mustang Mk IAs for their own use.Chorlton, p. 26. Fifty-five of these P-51-1s were outfitted with a pair of K.24 cameras in the rear fuselage for tactical low-level reconnaissance and re-designated F-6A (the "F" for photographic, although confusingly also still referred to as the P-51 or P-51-1). Two kept their P-51-1 designation and were used for testing by the USAAF.
Two XP-51s (serials and ) set aside for testing arrived at Wright Field on 24 August and 16 December 1941 respectively. The small size of this first order reflected the fact that what had been known as the USAAC up until late June 1941 was still a relatively small, underfunded peacetime organization. After the late-June 1941 reorganization of the USAAC into the United States Army Air Forces, roughly six months before the attack on Pearl Harbor changed the outlook for the United States regarding involvement in global hostilities against the Axis overnight, priority had to be given to building as many existing fighters – P-38s, P-39s, and P-40s – as possible while simultaneously training pilots and other personnel, which meant evaluation of the XP-51 did not begin immediately. However, this did not mean it was neglected, or testing and evaluation mishandled.Kinzey 1996, pp. 7, 17–18.
The 150 NA-91s were designated P-51 by the newly formed USAAF and were initially named Apache, although this was soon dropped and the RAF name, Mustang, adopted instead. The USAAF did not like the mixed armament of the British Mustang Is and instead adopted an armament of four long-barrelled 20 mm (.79 in) Hispano Mk II cannon, removing the .50 cal (12.7 mm) "nostril"-mounted weapons. The British designated this model as Mustang Mk IA, and would fit a number with similar equipment.Kinzey 1996, p. 18.
It was quickly evident the Mustang's performance, although exceptional up to (the supercharger's critical altitude rating), was markedly reduced at higher altitudes. This was because the single-speed single-stage supercharger fitted to the V-1710 had been designed to produce maximum power at low altitude; above that, power dropped off rapidly. Prior to the Mustang project, the USAAC had Allison concentrate primarily on turbochargers in concert with General Electric; the turbochargers proved to be reliable and capable of providing significant power increases in the Lockheed P-38 Lightning and other high-altitude aircraft, in particular in the Air Corps' four-engine bombers. Most of the other uses for the Allison were for low-altitude designs, where a simpler supercharger would suffice. Fitting a turbocharger into the Mustang proved impractical, and Allison was forced to use the only supercharger available. In spite of this, the Mustang's advanced aerodynamics showed to advantage, as the Mk I was about faster than contemporary Curtiss P-40 Warhawks using the same V-1710-39 (producing at , driving a diameter, three-blade Curtiss-Electric propeller).Gruenhagan 1980, pp. 178, 180–81. The Mk I was faster than the Supermarine Spitfire Mk Vc at and faster at , despite the British aircraft's more powerful engine (the Rolls-Royce Merlin 45, producing at .Birch 1987, p. 11.
Although it has often been stated that the poor performance of the Allison engine above was a surprise and disappointment to the RAF and USAAF, this has to be regarded as a myth; aviation engineers of the time were fully capable of correctly assessing the performance of an aircraft's engine and supercharger.Kinzey 1996, p. 8. As evidence of this, in mid-1941, the 93rd and 102nd airframes from the NA-91 order were slated to be set aside and fitted and tested with Packard Merlin (the US-built version of the Merlin) engines, with each receiving the designation XP-51B.Kinzey 1996, pp. 8, 18, 55.
P-51A/Mustang Mk II (NA-99)
On 23 June 1942, a contract was placed for 1,200 P-51As (NA-99s). The P-51A used the new Allison V-1710-81 engine, a development of the V-1710-39, driving a -diameter three-bladed Curtiss-Electric propeller. The armament was changed to four .50 in (12.7 mm) Browning machine guns, two in each wing, with a maximum of 350 rounds per gun (rpg) for the inboard guns and 280 rpg for the outboard. Other improvements were made in parallel with the A-36, including an improved, fixed air duct inlet replacing the movable fitting of previous Mustang models and the fitting of wing racks able to carry either drop tanks, increasing the maximum ferry range to with the tanks. The top speed was raised to at . The USAAF received 310 and the RAF 50 (as the Mustang II) before production converted to producing the Merlin-powered P-51B.Chorlton, p. 37; Gruenhagan 1980, pp. 66–67, 178, 182, 196.
A-36A (NA-97)
The A-36A was the first aircraft based on the 'Mustang' airframe ordered by the U.S. Government specifically for use by the USAAF. NAA found an unfilled 'Dive Bomber' USAAF contract, which they got mainly by their own initiative. In so doing, NAA was able to keep the production bays open with the hope the USAAF would place orders for it as a fighter. On 16 April 1942, Fighter Project Officer Benjamin S. Kelsey ordered 500 A-36As, a redesign that included six .50 in (12.7 mm) M2 Browning machine guns, dive brakes, and the ability to carry two 500 lb (230 kg) bombs. Kelsey would rather have bought more fighters but was willing instead to initiate a higher level of Mustang production at North American by using USAAF funds earmarked for ground-attack aircraft when pursuit aircraft funding had already been allocated."P-51 History: Mustang I." The Gathering of Mustangs & Legends. Retrieved: 26 March 2009. It was the first airframe of the Mustang "family" to be drop-tank capable.
The 500 aircraft were designated A-36A (NA-97). This model became the first USAAF Mustang to see combat. One aircraft (British serial EW998) was passed to the British who gave it the name Mustang Mk I (Dive Bomber).
Merlin-engined Mustangs
Mustang Mk X
In April 1942, the RAF's Air Fighting Development Unit (AFDU) tested the Mustang and found its performance inadequate at higher altitudes. As such, it was to be used to replace the P-40 in Army Cooperation Command squadrons, but the commanding officer was so impressed with its maneuverability and low-altitude speeds, he invited Ronnie Harker (from Rolls-Royce's Flight Test establishment) to fly it. Rolls-Royce engineers rapidly realized equipping the Mustang with a high altitude Merlin 61 engine with its two-speed two-stage supercharger would substantially improve performance. The company started converting five aircraft as the Mustang Mk X with the Merlin 65. Apart from the engine installation, which utilized custom-built engine mounts designed by Rolls-Royce and initially the -diameter four-bladed Rotol propeller from a Spitfire Mk IX,Gruenhagen, 1980, pp. 81–82. the Mk X was a straightforward adaptation of the Mk I airframe, keeping the same radiator duct design. The Vice-Chief of the Air Staff, Air Marshal Sir Wilfrid R. Freeman, lobbied vociferously for Merlin-powered Mustangs, insisting two of the five experimental Mustang Xs be handed over to Carl Spaatz for trials and evaluation by the U.S. Eighth Air Force in Britain.Bridgman 1946, pp. 248–249. The high-altitude performance improvement was remarkable: one Mk X (serial number AM208) reached at with full supercharger, and AL975 tested at an absolute ceiling of .Birch 1987, p. 35.
P-51B and P-51C
The two XP-51Bs were a more thorough conversion than the Mustang X, with a tailor-made engine installation and a complete redesign of the radiator duct. The airframe itself was strengthened, with the fuselage and engine mount area receiving more formers because of the greater weight of the Packard V-1650-3 compared with the V-1710. The engine cowling was completely redesigned to house the Packard Merlin, which, because of the intercooler radiator mounted on the supercharger casing, was taller and used an updraft carburetor, rather than the downdraft variety of the Allison.Gruenhagen 1980, pp. 72–73. The new engine drove a four-bladed -diameter Hamilton Standard propeller that featured cuffs of hard molded rubber.Gruenhagen 1980, p. 73. To cater for the increased cooling requirements of the Merlin, a new fuselage duct was designed. This housed a larger radiator, which incorporated a section for the supercharger coolant, and, forward of this and slightly lower, an oil cooler was housed in a secondary duct which drew air through the main opening and exhausted via a separate exit flap.Gruenhagen 1980, pp. 74, 76–77.
A "duct rumble" heard by pilots in flight in the prototype P-51B resulted in a full-scale wind-tunnel test at NACA's Ames Aeronautical Laboratory. This was carried out by inserting the airplane, with the outer wing panels removed, into the 16-foot wind tunnel. A test engineer would sit in the cockpit with the wind tunnel running and listen for the duct rumble. It was eventually found that the rumble could be eliminated by increasing the gap between the lower surface of the wing and the upper lip of the cooling system duct from . They concluded part of the boundary layer on the lower surface of the wing was being ingested into the inlet and separating, causing the radiator to vibrate and producing the rumble.Matthews 1943. The production P-51B inlet was lowered even further, to give a separation of from the bottom of the wing. In addition, the shelf above the oil cooler face was removed and the inlet highlight swept back.Lednicer, 1991.