The initiative to create the Thunderbolt II came from the U.S. Air Force, which in 1966 launched the program A-X (Attack eXperimental). It was planned to develop a vehicle capable of speeds of at least 650 km/h, with high maneuverability at low altitudes, the possibility of basing on unpaved airfields, and powerful cannon armament.
In 1972, two major American companies, Northrop and the Fairchild Republic, presented their prototypes. The first showed the YA-9A, for $29 million, the second showed the YA-10A, on which they spent $41.2 million.