The F-111 was designed by the General Dynamics Corporation as the TFX long-range supersonic variable wing sweep bomber for the USAF and US Navy in the early 1960s. In 1963, the Australian Government selected the aircraft to replace the Canberra bomber in RAAF service even though it had yet to be flown. The protracted design and flight-testing program caused intense criticism. The first F-111C was accepted by the RAAF in 1968 but fatigue cracking of the wing carry-through box which was part of the variable wing sweep mechanism meant that the first F-111 was not delivered to Australia until June 1973. The RAAF had to lease 24 F-4E Phantom jets from the USAF in the interim. Since 1973, the F-111 has served with Nos 1 and 6 Squadrons at Amberley RAAF Base in Queensland. The USAF retired the last of its F-111 aircraft in the late 1990s leaving Australia as the sole remaining user of the type. It is scheduled to be withdrawn from RAAF service in 2010-11.