The Fairchild Republic A-10 was developed in the 1960s and 1970s, even as American helicopters were being shot down in Vietnam with frightening regularity. It was the first airplane designed specifically for close air support, with the goal of defending soldiers against artillery, tanks and other weapons.
It was basically designed to “take apart a Soviet tank,” says Jeffery S. Underwood, a historian at the United States Air Force Museum at Wright-Patterson Air Force Base in Ohio. To that end, the A-10 typically is equipped with the AGM-65 Maverick air-to-surface missile and is capable of carrying many other armaments, including AIM-9 Sidewinder air-to-air missiles.
But its primary weapon is a seven-barrel GAU-8 Avenger Gatling cannon. It measures 9 feet long and fires 30mm armor-piercing shells which are held in a drum not quite six feet in diameter. It can spit them out at a rate of 3,900 rounds per minute, and accounts for some 16 percent of the plane’s unladen weight. The gun is so large and so integral to the A-10, that the airplane is effectively built around it. In fact, when the gun is removed for maintenance, the tail of the plane must be supported to keep it from falling over.
http://www.wired.com/2014/12/a10-warthog-isis/
Interesting how this has kept going while newer tech like the boeing-sikorsky rah-66 comanche never made it into production...