A rotary cannon or Gatling gun is a rapid-fire weapon the utilizes a rotating cluster of multiple barrels (hence the name) to fire projectiles or bullets. This feature allows a higher fire rate than single barreled machine guns or automatic cannons. The rotating feature is done simultaneously with the loading and firing of different barrels, which also allows some time for the barrel to cool, allowing sustained fire for longer periods than other automatic weapons.
The original rotary cannon, the Gatling gun, was the forerunner to the modern-day rotary cannon used today. Designed in 1861 by Dr. Richard J. Gatling and patened in 1862. It was hand-cranked and hopper fed, which allowed it to fire at 200 rounds per minute. The Gatling gun was used in the field, first in the American Civil War and subsequently by European and Russian armies.
After Gatling guns were rendered obsolete due to their weight in 1911, the United States Army Air Force and later the United States Air Force after World War II needed a automatic cannon able to fire quickly enough to strike fast-moving enemy jets. They used experience from the 20mm and 30mm Gatling guns used previously by the German Luftwaffe.
In June 1946 General Electric was awarded an Defense contract to develop an aircraft gun with a high rate of fire which General Electric named Project Vulcan. This would later develop into the M61 Vulcan.