The YA-9 competed with the YA-10 during the AX program for a new Ground-Attack aircraft for the US Air Force. A low cost aircraft was to be created by 6 March 1967, the day on which the US Air Force released a request for information to 21 defense contractors. After discussing and analyzing of Vietnam missions, the situation called for an aircraft being able to take-off from a runway with a length of only 300m, the aircraft should need fully loaded only 300 m to perform a 180° turn at a speed of 300 km/h and the maximum flight endurance was not to be lower than five hours. The cost of the aircraft was set to 1.4 million US-dollar.
The YA-10A and the YA-9A were considered as the two winners on 18th May 1970, two prototypes were built of each aircraft, to be used in a competitive fly-off. Northrop received 28.9 Million US-dollar for this contract. The YA-9 prototypes were built in Hawthorne and were first flown on 30th May 1972 (71-1367 flown by Lew Nelson) and on 23rd August 1972 (71-1368) in Edwards.
Finally the YA-9 was inferior to the YA-10, which was later developed into the A-10 Thunderbolt II; the decision was made on 18th January 1973.