The next Army contract for Mustangs consisted of an order on August 24, 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. The first P-51A flew on February 3, 1943, and the first deliveries began the next month. 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 inboard pair had 350 rpg and the outboard pair had 280 rpg. An underwing load consisted of two 250 lb, 325, or 500-pound bombs. Maximum takeoff weight rose to 10,600 pounds, but maximum ferry range was increased to 2350 miles. The P-51A had the Allison V-1710-81 (F20R) engine rated at 1200 hp for takeoff and 1125 hp at 18,000 feet, with significantly better high-altitude performance than the V-1710-39 of the P-51. The engine was fitted with a new supercharger which further enhanced low-altitude performance. In addition, a larger-diameter propeller was fitted. Maximum speed rose to 409 mph at 11,000 feet, faster at medium altitudes than any other fighter then in service.
Because of the thin wing cross section, the wing guns lay almost on their sides and the ammunition belt feeds had to be built with some rather sharp kinks in them in order to direct the bullets into the guns. This awkward arrangement resulted in many gun jams, particularly after maneuvers in which high g-values were pulled.
Three production blocks were built with the following serials:
43-6003/6102 P-51A-1-NA 43-6103/6157 P-51A-5-NA 43-6158/6312 P-51A-10-NAOf the 310 P-51As built, 35 of them were fitted with twin-K24 camera installations and had their guns removed. These were redesignated F-6B.
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 USAAAF F-6As. The RAF serials of the Mustang IIs were FR890/FR939. Deliveries were made late in 1942. Mustang II FR901 was fitted with special deep-section fuel tanks beneath the wings for ultra-long-range flying. FR893 was tested at Boscombe Down, and demonstrated a best rate of climb of 3800 feet per minute at 6000 feet, with n altitude of 20,000 feet being reached in 6.9 minutes and 34,000 feet in 24 minutes. 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.
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. On November 25, 1943, the 530th Fighter-Bomber Squadron of the 311th Fighter Bomber Group flew the first of the Mustang's long-range escort missions, using drop tanks to escort B-24 Liberators in an attack on Rangoon, Burma, a round trip of nearly 900 miles.
The F-6Bs, on the other hand, served in Europe, mainly with the 107th Tactical Reconnaissance Squadron based in England.
When production of the Allison-engined Mustang ended, 1580 examples had been built.
Specifications of P-51A-10-NA:
One 1200 hp Allison V-1710-81 twelve-cylinder Vee liquid cooled engine. Performance Maximum speed was 340 mph at 5000 feet, 360 mph at 10,000 feet, 380 mph at 15,000 feet, and 390 mph at 20,000 feet. Range on internal fuel was 750 miles at 300 mph at 10,000 feet. Range with two 125 Imp. gall. drop tanks was 2000 miles at 266 mph and 2350 miles at 228 mph. An altitude of 5000 feet could be attained in 2.2 minutes, 10,000 feet in 4.4 minutes, and 20,000 feet in 9.1 minutes. Service ceiling was 31,350 feet. Weights: 6433 lbs empty, 8600 lbs normal loaded, and 10,600 lbs maximum loaded. Dimensions: Wingspan was 27 feet 0 1/4 inches, length was 32 feet 2 1/2 inches, height was 8 feet 8 inches, and wing area was 233 square feet. Armament: Four 0.50-inch machine guns mounted in the wings. The inboard pair had 350 rpg and the outboard pair had 280 rpg. An underwing load consisted of two 250 lb, 325, or 500-pound bombs.