The YA-9 took its first flight on 30 May 1972, with the second prototype flying on 23 August. Northrop's flight testing was successful, with the aircraft claimed to have "fighter-like" handling and to be a good weapon platform. A fly-off by USAF test pilots of the two competing designs took place between 10 October and 9 December 1972. While the YA-9 fully met the USAF's requirements, the YA-10 was declared the winner on 18 January 1973. The use of the established TF34 engine rather than the untried F102 by the YA-10 may have been preferred by the Air Force, while Fairchild had no alternative work available and was unlikely to survive if it did not win the A-X contract.
The two YA-9 prototypes were subsequently relegated to NASA for continued flight testing before being retired. When retired, the YA-9s' custom-built engines were removed and were later mated to a C-8 Buffalo airframe as part of the NASA-Boeing joint Quiet Short-haul Research Aircraft (QSRA) study into a quiet short-haul commercial aircraft.