The Republic XF-103 was an American project to develop a powerful missile-armed interceptor aircraft capable of destroying Soviet bombers while flying at speeds as high as Mach 3. Despite a prolonged development, it never progressed past the mockup stage. Development In 1949, the USAF issued a request for an advanced supersonic interceptor to equip the Air Defense Command. Known formally as Weapon System WS-201A, but better known informally as the 1954 interceptor, it called for a supersonic aircraft with all-weather capability, powerful airborne interception radar, and air-to-air missile armament. Republic was one of six companies to submit proposals. On 2 July 1951, three of the designs were selected for further development, Convair's scaled-up XF-92 that evolved into the F-102, a Lockheed design that led to the F-104, and Republic's AP-57. AP-57 was an advanced concept to be built almost entirely of titanium and capable of Mach 3 at altitudes of at least . A full-scale mock-up of the AP-57 was built and inspected in March 1953. A contract for three prototypes followed in June 1954. Work on the prototypes was delayed by continued problems with the titanium construction, and more by continuing problems with the proposed Wright J67 engine. The contract was later reduced to a single prototype. In the end, the J67 never entered production and the aircraft it had been chosen for were forced to turn to other engine designs, or were cancelled outright. Republic suggested replacing the J67 with the Wright J65, a much less powerful engine. The project was eventually cancelled on 21 August 1957 with no flying prototypes ever being completed. The design was given a brief reprieve as part of the Long-Range Interceptor – Experimental (LRI-X) project that ultimately led to the North American XF-108 Rapier. Part of this project was the development of the advanced Hughes AN/ASG-18 pulse-doppler radar and the GAR-9 missile. Republic proposed adapting the F-103 as a testbed for these systems with additional fuel tanks taking up much of the original weapon bay spaces, although it wouldn't be able to come close to meeting the range requirements of LRI-X. Some work was carried out adapting the mockup to house the 40 inch antenna, which required the nose section to be scaled up considerably. Nothing ever came of the proposal, and testing of the ASG-18/GAR-9 was carried out on a modified Convair B-58 Hustler instead. Design Propulsion Mach 3 performance in