The Northrop YA-9 was a prototype attack aircraft developed for the United States Air Force A-X program. A single 30 mm rotary cannon was to be fitted in the belly of the aircraft, with the gun barrels extending under the nose. As the gun was mounted on the aircraft's centerline, the undercarriage nosewheel was offset one foot (0.30 m) to the left. As the GAU-8 Avenger cannon was not ready, both YA-9 prototypes (and the two YA-10s) were fitted with the smaller 20 mm M61 Vulcan instead. Ten underwing hardpoints were fitted, allowing up to 16,000 lb (7,300 kg) of weapons, including bombs and AGM-65 Maverick air-to-ground missiles, to be carried.

The YA-9 took its first flight on 30 May 1972,[1][14] with the second prototype flying on 23 August.[13] Northrop's flight testing was successful, with the aircraft claimed to have "fighter-like" handling and to be a good weapon platform.[15] A fly-off by USAF test pilots of the two competing designs took place between 10 October and 9 December 1972.[16] 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 by the YA-10 rather than the untried F102 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.[13]

The two YA-9 prototypes were subsequently relegated to NASA for continued flight testing before being retired.[13] 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.

The Fairchild Republic YA-10 that entered production as the A-10 Thunderbolt II.

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