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. Out of 150 produced, only 93 ended up going to the RAF, with 55 being retained by the USAAF and two being devoted to the XP-78 (XP-51B) project. These included RAF Mustang IA serials FD418/FD437, FD450/FD464, FD466/FD469, and FD510/FD527.
Contrary to some reports, the Army planes still retained the 20-mm cannon of the RAF version, but were fitted with two K-24 oblique cameras mounted behind the pilot in the fuselage. These were designated as tactical reconnaissance aircraft and were designated F-6A, but this designation was soon changed to P-51. Some sources say that the F-6As had an additional downward-pointing fuselage camera munted between the tailwheel bay and the radiator exit flap, with the two cameras mounted behind the pilot being common to both designations.
The Army planes were delivered directly from NAA and were all finished in OD/Neutral Gray camouflage. The P-51s went to Peterson Field in Colorado, where they were assigned to the newly-established aerial reconnaissance school. 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 10, 1943, which was the first USAAF Mustang mission of the war. No. 225 Squadron of the RAF frequently borrowed Mustangs from the 154th to augment its shorter- range Spitfires. The F-6A/P-51 was quite successful in operation, but it did have one important defect--it bore a similar shape to that of the Messerschmitt Bf 109. The 154th's first combat loss was a friendly fire incident in which Allied AAA failed to recognize the differences, with fatal results.
Two P-51 airframes were diverted to the XP-78 project, about which
much more will be said later!