Soon after the end of the Second World War the newly formed United States Air Force identified a need for an improved gun system for its aircraft. While adequate as an air combat / ground attack weapon during World War Two, the Browning M2 .50 caliber machine gun had been rendered obsolete by jet aircraft and needed to be replaced by a weapon with increased range, rate of fire, and projectile lethality. Realizing that singled barreled automatic weapons had essentially reached their design limits, the U.S. Army Ordnance Research and Development Service hit on the idea of re-introducing the multi-barreled rotary weapon invented by Richard J. Gatling in the 1880s. Initial tests proved promising as a vintage Gatling gun, now powered by an electric motor in place of the usual hand crank, was able to achieve rates of fire in excess of 4,000 rounds per minute.
In 1946 the General Electric company received the contract for this new program, code named "Project Vulcan," and was tasked with producing functional prototypes in a number of calibers for further testing. In 1952 GE produced three different guns; .60 caliber, 20mm, and 27mm. After extensive testing, the 20mm version was selected for further testing to determine its suitability as an aircraft mounted weapon. In 1956 the gun was standardized as the M61 20mm cannon and entered service with both the United States Army and Air Force.
The current version of the M61, the M61A1, remains relatively unchanged from past models. In its basic form the M61A1 is a six barreled rotary cannon firing a variety of electrically primed 20mm cannon shells. The gun can be mounted either internally in aircraft, or externally in a pod arrangement, and has been mounted on both ground vehicles and trailers for use as an air defense weapon. The M61A1 can be driven hydraulically, electrically, or by ram air and has a variable rate of fire of between 4,000 and 7,200 rounds per minute (depending on individual settings and performance requirements).