Unique to the P-51 was the laminar flow wing design which was developed by the US National Advisory Committee for Aeronautics (NACA). Contemporary aircraft wings featured a wing cross-section with maximum thickness about a fifth of the way across the wing from the leading edge with most of the camber on top of the wing. The laminar flow wing in contrast has its maximum thickness well aft from the leading edge and has almost as much camber on top as on the bottom. This feature reduced turbulent flow across the wing, thereby reducing drag and increasing speed and range. Drag was also reduced by positioning a ventral radiator underneath the rear of the fuselage to present the smallest possible fuselage cross section.
During the design stage, on May 4, 1940, the US Army released the design for export with the condition that two of the planes be delivered to them for evaluation. At this time, the NA-73 was assigned the XP-51 designation. The first and tenth airframes were sent to the Army for testing and were given the serial numbers 41-38 and -39. An order for 150 P-51s followed which was to satisfy the RAF request as part of the Lend Lease legislation. After Pearl Harbor, fifty-three of these were kept back as reconnaissance aircraft. Initially, the P-51 was named "Apache" for a short time, but the name "Mustang" was later adopted. The British designation would be Mustang I. Most of the first 20 Mustangs to arrive in England were used for test and evaluation.