In 1940, the US Army had given its permission for the initial British Mustang delivery to proceed, with the proviso that two of the NA-73s destined for England be made available to the Army for tests free of charge. In a separate contract dated September 20, 1940, the two aircraft delivered to the Army were to be the fourth and tenth production NA-73s, and the planes were to be designated XP-51.
The fourth and tenth NA-73s were duly delivered to the US Army in May of 1941 for testing at Wright Field, Ohio. They bore the designation XP-51 and were assigned the serial numbers 41-038 and 41-039. They were initially unpainted except for national insignia and the black antiglare panel over the forward fuselage ahead of the pilot. The Army painted the serials 1038 and 1039 on the fin and on each side of the nose, together with the WRIGHT arrowhead emblem on the rear fuselage. Much later, they were both painted olive drab overall.
The testing of the two Wright Field XP-51s was rather slow at first, almost as if the Army didn't really want to bother with these airplanes and that they were some sort of nuisance that the Army wished would just go away. Some authors have suggested that there were dark and evil motives behind the Army's reluctance to test the XP-51s. However, the slow pace of the testing of the XP-51s can probably be blamed more on bureaucratic inertia than on anything all that sinister. At that time, the Army was overloaded with other test programs, the Lockheed P-38 Lightning, Bell P-39 Airacobra, and Republic P-47 Thunderbolt being thought to meet all the Army's requirements for fighter aircraft. Furthermore, the Mustang was a "foreign" type not built to any American specification, and was therefore way down on the Army's list of priorities.
Nevertheless, the testing of the XP-51s did eventually get underway at Wright Field, and the Army's test pilots reported very favorably on the performance of these planes. Inexplicably, no Army orders were forthcoming. Much later, the Senate Special Committee to Investigate the National Defense Program (better known as the "Truman Committee", after its chairman Sen. Harry S Truman of Missouri) was given the task of investigating the system under which military production contracts were awarded during wartime conditions. They looked specifically into the reason why the Army had sat on its hands for so long before ordering any examples of the Mustang, an airplane which had such demonstrably superior performance. Some insiders claim that NAA President "Dutch" Kindelberger had been asked to pay bribes in exchange for a production contract, but that he had refused all such demands in no uncertain terms. The primary cause of the long delay in Army acquisition of the Mustang may be somewhat less sinister. The Mustang may have been the victim of the "Not Invented Here" (NIH) syndrome, in which the Army looked askance at an upstart aircraft which had not been designed in response to any of its official requirements.