An Inspector looks over a Mustang Mk I fighter

An inspector with North American Aviation in Inglewood, California, looks over a Mustang Mk I fighter destined for the British Royal Air Force in fall 1942. 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 Mustangs to be sent to Brittan. This allocation was NA-91, RAF designation of Mustang Ia. The Mustang Ia was equipped with four Hispano 20mm cannons installed in the wings. The nose guns were deleted. Out of the 150 ordered, only 111 were serialed for the RAF and probably less than that actually received. After the attack of Pearl Harbor, the US Army held the remaining Lend/Lease order of NA-91s to Brittan. These, about 55, were designated P-51 and were fitted with four .50 cal guns instead of the cannons. But not all were configured with the Brownings. Cameras were added and a new designation of F-6A. The US Army actually called the NA-91s "Apache".

Source ;
Library of Congress, Office of War Information, 1a35319u
Smithsonian National Air and Space Museum
https://www.flickr.com/photos/airandspace/albums/72157715574200936
https://www.mustangsmustangs.net/p-51/variants/raf_mustangs