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.
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. RAF serials were AL958/AL999, AM100/AM257, and AP164/AP263. 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.
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. The RAF serial numbers assigned to this lot were FD418/FD567. For contractual purposes, these aircraft were assigned the US designation of P-51, and the Allison V-1710-F3R engine was given the US Army designation V-1710-39. The P-51s were assigned the USAAF serials 41-37320/37469.
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.
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 fitted with two K-24 cameras in the fuselage, and they retained the four 20-mm cannon of the RAF version. 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.
P-51 41-37426 was given to the US Navy for evaluation (BuAer #57987). The Mustang was found to be unacceptable because of a poor rudder control at low airspeeds, particularly at high angles of attack.