The A-10 Thunderbolt is most known for it's GAU-8 Avenger 30 mm cannon, that makes a distinguishable "brrrrt" sound.
The GAU-8 was special made for the A-X program, with a full scale size larger than a Volkswagon Beatle.
The YA-9 was the Northrop entry in the A-X program which lost to the YA-10, the plane that would eventually become known as the Thunderbolt.
Many Soldiers that have returned from the front lines will tell you of the importance of close-air-support (CAS). The A-1 Skyraider fulfilled this role in the Korean War, entering service a year too late for World War II. By the Vietnam War, the F-4 Phantom II had adopted this role with its heavier payload and faster speeds. By the time the Vietnam War ended, the Air Force was looking for a better solution to provide CAS to ground forces while allowing an aircraft to break deep into enemy territory to attack a target.
The Air Force was also concerned about the Phantom's inability to stay near ground forces, as jet engines meant the aircraft had poor staying power. Attack helicopters were considered as well, but the poor armoring and limited payload made them insufficient. The result was the A-X Program, ordered on 8 September 1966. By 1972, two prototypes were ready to fly, both of which were built around the GAU-8 30 mm cannon. The first of these was the Northrop YA-9A. The second was what would become the A-10 Thunderbolt II - the "Warthog." The plane first flew against the YA-9 in May of 1972, and with various testing and revisions to the fire control systems, was introduced finally in October of 1977.
Designed to withstand a beating, the Warthog is capable of surviving direct hits from armor-piercing and high explosive projectiles. It is even capable of withstanding a direct hit from surface-to-air missiles (SAMs). The hydraulic system is redundant, enabling manual override of the hydraulics if the system fails. The pilot is protected in the cockpit by heavy duty armor. Called "the tub", the cockpit has been tested against rounds up to 57 mm. The GAU-8 cannon, and the sound of the two GE TF-34 turbofans are signature sounds that any Soldier who has been at the front will tell you "sounds like an angel." With the ability to attack deep into enemy territory, protect forces on the front line, and return with only half of its airframe intact, it is no wonder that the Warthog has remained in service by demand of all military services.
The signature sound of the GAU-8 30 mm cannon is the calling card of the A-10. Designed to be specifically fitted to the winner of the A-X Program, the GAU-8 gun was designed to be an aerial ordnance system. The idea and concept of the aerial ordnance and artillery platform originated from a U.S. Army Weapons Command (WECOM) concept generated in the 1950s which called for both a fixed wing, variable wing, and rotary wing artillery weapons platform. These evolved into the A-10 Thunderbolt, the V-22 Osprey and AC-130 Gunship, and the AH-64 Apache designs - respectively. The GAU-8 Avenger was longer than a Volkswagen Beatle automobile and was about as tall. The GAU-8 composes the majority of the total weight of the A-10, and is the anchor for the planes center of gravity.
When the A-10 was in it's prototype phase, the Avenger was not yet available. As a result of this, Fairchild engineers substituted the 30 mm Avenger with the 20 mm Vulcan. When the plane entered production, all planes were fitted with the Avenger we know of today. The next largest weapons system, the Vulcan, appears on the F-16 Falcon.
To date, no other aircraft use a full-sized GAU-8 system. Even the AC-130 only uses a partial system, saving weight for larger artillery pieces on-board. The A-10 is the only aircraft to deploy the original GAU-8 in it's fully functional size.