P-51 fighter planes being prepared for test flight at the field of the North American Aviation

Image size: 1600 x 1229 pixel. 488 KB
Date: October 1942
Place: Inglewood, California, USA
Photographer: Alfred T. Palmer from Office of War Information

North American NA-91 Mustang fighters being serviced at North American Aviation at Inglewood, California (USA), in October 1942. After passing of the lend-lease act in March 1941, the USAAF ordered 150 NA-93 Mustang Mk IA fighters on 25 September 1941 for delivery to the United Kingdom. The RAF serial numbers assigned were FD418-FD567 (FD553 is visible on the left). For contractual purposes, these aircraft were assigned the U.S. designation of P-51 (USAAF serials 41-37320 to 41-37469). The Mustang IA differed from earlier versions in having the machine guns replaced by four 20 mm wing-mounted Hispano cannon. After December 1941 serials FD418-FD437, FD450-FD464, FD466-FD469, and FD510-FD527 were reposessed by the USAAF (and briefly named A-36A Apache). These Mustangs will later be fitted with their designed armament of 4 x 20mm cannon. The British designated this model Mustang Mk IA To the USAAF is was the P-51 (Exactly that, no sub-type suffix letter or production block number.) Note that the machine at left has an RAF serial number and fin flash but a US star on the fuselage. That aircraft and the one at right appear to be in RAF camo whereas the other two seem to be in Olive Drab indicating that they are destined for the USAAF. North America's development and production of their NA-73 design for the UK was approved after 2 examples were supplied to the USAAC as XP-51, but before that the US army had already contracted for the procurement of 150 additional aircraft for supply to the UK under lend-lease, designating these P-51 and they differed from the initial eight gun armament by having four wing mounted 20mm canon (as pictured). From this batch 93 went to the UK becoming Mustang IA 55 went to the USAAF as F-6A's and the remaining 2 were also taken on by the USAAF and re-engined as XP-51B's. Not the first models to enter combat for the US, those were A-36A's in August 1943 (P-51 with dive brakes, bomb racks and six 0.50in) which saw service in the Middle East and in the invasion of Sicily

Source:

Library of Congress LC-USW361-495 

http://en.wikipedia.org/wiki/File:P-51_%28mustang%29.jpg