P-51 (Mustang IA)

Lend/lease act

New guns

US interest starts


Serial numbers

3-Way drawings



Lend/lease act

In the beginning of Mustang development, there was little direct interest and involvement by the USAAC. In a contract approved on September 20th , 1940, thanks to First Lieutenant Benjamin S. Kelsey, head of the Army Air Corps Pursuit Projects Office at Wright Field, it was agreed that the fourth and tenth production NA-73s would be the planes diverted to the Army for testing and evaluation. NAA received no payment for these two aircraft, but the possibilities of sales to the USAAC made it worthwhile.
Those 2 Mustangs would be designated XP-51 (41-038 & 41-039). The first XP-51 was flown in late may 1941 but didn't make it to the USAAF (the "Air Corps" was renamed "Air Forces" in June 1941) testing grounds at Wright Field , Ohio , until August 24th, 1941. The second arrived on December 16 th .

XP-51 41-039
The second XP-51 with serial 41-039 (photo courtesy of USAF)

Although the evaluation was very positive, the USAAF was still slow to take an interest in the Mustang. Because of the still increasing threat of global war, top priority was given to the already existing production lines of fighter aircraft such as the Lockheed P-38 and not to testing of new aircraft. As a result, both XP-51s were put in storage at Wright Field , Ohio .

Meanwhile the British were already finding use for their Mustang Is in the reconnaissance and ground attack role. The Mustang was so promising that in late 1941 the RAF ordered another 300.

In March of 1941, the US Congress passed the Lend/Lease Act which permitted the "lending" of US built aircraft to nations that were "vital to the security of the United States ". This allowed the US to place an order for 150 more Mustangs to be sent to Brittan. As the exigencies of war demanded, 93 of these 150 (factory designated NA-91) ended up in British service. USAAC designated the type as P-51 whilst the British named their new acquisition "Mustang IA". The Mustang IA also carried a new evolution Allison V-1710-F20R (-81) engine, producing 50 more horsepower.

Allison V-1710-F20R


New Guns

The Mustang IA was equipped with four Hispano 20mm cannons, two in each wing. The nose guns were deleted. This unusual armament (the Mustang IA was the only variant of the type with this gun layout) was installed at the request of the British who had been plagued with a serious lack of firepower during the Battle of Britain. The cannons protruded distinctly out in front of the each wing.

Courtesy of In Action 45 - P-51 Mustang by Squadron/Signal Publications

When the RAF asked to to test a new pneumatic gun charger, P-51 41-038 was dragged out for use as a testbed. In late 1941, the USAAC finally found out what the RAF already knew: the test pilots came back talking only of the hot new fighter and not of the gun charger they were supposed to test.


US interest starts

USAAC finally began to show interest and were surprised to find out they had actually already purchased 150 of them (designation P-51 Apache) because of the Lend Lease program. They were actually Mustang IAs.

After the attack of Pearl Harbor , the US Army held the remaining Lend/Lease order of NA-91s to Britain. These, about 55, were designated P-51. Mustangs with RAF serials FD418/FD437, FD450/FD464, FD466/FD469, and FD510/FD527 were diverted from the RAF order and delivered to the US Army.

The Army planes were fitted with two K-24 cameras in the fuselage. Some were fitted with four 0.50-inch machine guns, but most retained their quartet of 20-mm cannon. Most also 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. One was converted as P-51-1 (42-37320), and the rest were converted as P-51-2. The serial numbers of the Mustang IAs that were diverted to the USAAF were 41-37320/47339, 41-37352/37366, 41-37368/37371, and 41-37412/37429.

93 Mustangs were delivered to the RAF and 55 were redesignated to the USAAF. Of the initial 150 ordered, this means there were 2 Mustangs left. These two Mustangs were again held back by the US for eventual testing with the Merlin engine, also known as the XP-78 project (more about this in the P-51 B/C section). This is (again) a clear sign that the US understood the value of the design and was planning ahead for future use of the fighter.

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 10th, 1943.

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