On November 16, 1970, the Air Force issued a request for a proposal for a 30mm rapid fire cannon to use in the A-X (The development designation for the A-10) Close Air Support aircraft. In June of 1971 General Electric and Philco Ford were selected to build the prototype guns, designated GAU-8.
The contracted specifications directed the gun be capable of destroying a wide variety of targets expected to be encountered during a close air support mission, such as light, medium and heavy tanks, armored personnel carriers, and fixed or mobile artillery. The specifications also called for the gun to be capable of destroying hardened targets like bunkers and equipment within revetments.
The first in-flight testing of the GAU-8 was done on February 26, 1974. The gun was fired for the first time in flight with combat ammunition on June 19, 1974. The Avenger fired more than 39,000 rounds of ammunition during approximately 60 test flights. The gun was tested in a wide variety of flight profiles including testing from as high as 25,000 feet to as low as 100 feet, testing at speeds ranging from 135 knots to 415 knots, and testing in all attitudes (various roll, pitch and yaw) with up to five G’s.
The test program went smoothly and the first test mission cancellation didn’t occur until November 12, 1975, when the gun jammed. The test program identified a few problems including gun residue buildup on the canopy and problems with gun gas ingestion into the engines. Both problems were solved and fixes were incorporated into production aircraft beginning with aircraft No. 16 (earlier aircraft were retrofitted with the fixes). The “Avenger” was installed in all production A-10s and retrofitted to both YA-10As.