The definitive version, the P-51D, was powered by the Packard V-1650-7, a license-built version of the Rolls-Royce Merlin 60 series two-stage two-speed supercharged engine, and armed with six .50 caliber (12.7 mm) M2 Browning machine guns.
Prototipni NA-73X so predstavili 9. septembra 1940, 102 dneva po podpisu pogodbe; z naknadno vstavljenim motorjem je prvič poletel 26. oktobra.