North American P-51 Mustang P-51A-NA (NA-99)

On 23 June 1942 a contract was placed for 1,200 P-51As, designated NA-99 by North American. The model introduced the Allison V-1710-81 engine, a development of the V-1710-39, driving a 10 ft 9 in (3.28 m) diameter three-blade Curtiss-Electric propeller. Armament was simplified to four .50 in (12.7 mm) Browning machine guns, two in each wing, with a maximum of 350 rounds per gun for the inboard weapons and 280 for the outboard ones.

Other improvements were made in parallel with the A-36: an improved fixed air duct inlet replacing the movable fitting of earlier Mustangs, and wing racks able to carry 75 or 150 US gal (280 or 570 L) drop tanks, the larger pair raising the maximum ferry range to 2,740 mi (4,410 km). Top speed rose to 409 mph (658 km/h) at 10,000 ft (3,000 m), the best figure reached by any Allison-engined Mustang.

Production fell well short of the original contract, since the lines soon converted to the Packard Merlin-powered P-51B. The sources consulted differ slightly over how the delivered aircraft were split: the variants article states that the USAAF received 310 and the RAF 50 — the latter as the Mustang II — while the production table records 310 airframes built at Inglewood, of which 50 were Lend-Leased to the RAF under that same name. Either way, the P-51A was the model whose potential Major Thomas Hitchcock was defending when he pressed for attention to the Merlin Mustang project.