Often depicted as a potential escort for the XB-70 Valkyrie, the F-108 was more intended as a long range interceptor for defending US airspace against projected Soviet supersonic bombers. The Rapier did show a considerable family resemblance to the Valkyrie – unsurprising, considering that both aircraft were to operate at speeds of Mach 3 at 80,000 feet.
There were, of course, many preliminary F-108 design iterations, but basically the Rapier would have been a large delta winged aircraft powered by a pair of General Electric J93s, with the crew of two seated in tandem. North American was contracted to build a pair of prototypes in 1957, but the program was cancelled in September 1959, well before either example was complete.
Lloyd S. Jones U.S. Fighters ARMY-AIR FORCE 1925 to 1980s Aero Publishers, 1975 p.292: scale XF-108 3-view plans