When the Air Force started looking for a new dedicated ground attack plane in the late 1960's, they first polled pilots of the venerable Douglas A-1 Skyraider who flew attack missions in Korea and Vietnam. Then they looked at the Ilyushin Il-2 Sturmovik from WWII, and the Henschel Hs 129, nicknamed the Panzerknacker (tank cracker). The Air Force determined that the ideal aircraft should have "long loiter time, low-speed maneuverability, massive cannon firepower, and extreme survivability." The A-X program would search for a new plane to fit these criteria.
In 1970, the Air Force issued a request for proposals, in large part to counter the threat of Soviet armored forces. Included in the new requirements was that the aircraft would be designed specifically around a 30mm cannon, have a maximum speed of 460 mph, a takeoff distance of 4000 feet, an external load of 16,000 pounds, a mission radius of 285 miles, and a cost of $1.4 million per plane. Northrop was chosen to build the YA-9, and Fairchild Republic was chosen to build the YA-10.
Northrop's contender for the contract was a traditional design, a plane which bears a certain resemblance to the Russian Sukhoi Su-25, NATO reporting name "Frogfoot". But then again, form does follow function. The National Museum of the Air Force writes about the YA-9:
The A-9A was a high-wing, twin-engine, single-place aircraft. The plane was designed for exceptional maneuverability and had large flight control surfaces. The engines were specifically designed for the project by Lycoming. Each of the YF102-LD-100 turbofans developed 7,500 pounds of thrust at maximum power. Offensive firepower consisted of a 20mm Vulcan cannon and up to 16,000 pounds of ordnance carried on ten external wing stations. The winner of the A-X fly-off would incorporate the 30mm GAU-8 cannon into production aircraft, but the gun was under parallel development during the A-X competition and wasn't ready for flight testing during the fly-off between October and December 1972. The A-9A featured a triple redundant hydraulic system, foam filled self-sealing fuel tanks and armor plating protecting vital systems including a titanium "bathtub" surrounding the cockpit (note: an aluminum "bathtub" was fitted in the prototype).
A fly-off between the YA-9 and YA-10 took place between October 9-December 10, 1972, and the YA-10 was declared the winner. Factors leading to the choice of the YA-10 were the less conventional engine placement which led to higher survivability in the case of a hit on the engine area, and the double tail, which helps conceal the engines' heat signature, as well as providing redundancy if one of the tails is shot away.
Two prototypes were built by Northrop, and after the competition they were given to NASA for further flight testing. The custom-built Lycoming engines were stripped from the aircraft and mated to a C-8 Buffalo as part of the research into a quiet, short-haul aircraft. One of the prototypes is on display at March Field Air Museum, and the other awaits restoration at Edwards Air Force Base in California.