The North American XF-108 Rapier was a proposed long-range, high-speed interceptor aircraft designed by North American Aviation intended to defend the United States <!---and Canada----sorry, Buttler is referring to a later "selling job" that was never the intention of the original project---> from supersonic Soviet strategic bombers. The aircraft would have cruised at speeds around with an unrefueled combat radius over , and was equipped with radar and missiles offering engagement ranges up to against bomber-sized targets.
To limit development costs, the program shared engine development with the North American XB-70 Valkyrie strategic bomber program, and used a number of elements of earlier interceptor projects. The program had progressed only as far as the construction of a single wooden mockup when it was canceled in 1959, due to a shortage of funds and the Soviets' adoption of ballistic missiles as their primary means of nuclear deterrence. Had it flown, the F-108 would have been the heaviest fighter of its era.
Prior to the project's cancellation, U.S. President Dwight D. Eisenhower noted that raising the F-108 interceptor force would have cost the U.S. taxpayer $4 billion (equivalent to $ billion today).
Development
LRI-X
During the early 1950s, the USAF proposed a very high-performance, long-range interceptor. On 20 July 1955, formal development of what became known as the Long-Range Interceptor, Experimental (LRI-X) was approved, planned as an F-102 Delta Dagger/F-106 Delta Dart replacement. It was to have a two-man crew and at least two engines.
Of the eight interested companies, contracts for preliminary studies were issued to North American Aviation, Lockheed and Northrop on 11 October 1955, five days after the specification's release. Political and budgetary difficulties led to the cancellation of the program on 9 May 1956.
WS-202A
After considerable confusion, the program was reinstated on 11 April 1957 with North American awarded a contract for two prototypes. The designation F-108 was issued, also known as "Weapon System 202A" (WS-202A). North American's company designation was "NA-257", although it was basically identical to the NA-236. At the time, Air Defense Command anticipated an order for 480 aircraft.
The resulting design went through considerable evolution, owing to both its cutting-edge technology and continual redefinition of the USAF requirements. Early revisions prominently featured canards, with a span of , and a wing of 53.5° sweep.
From September 1958, substantial engineering and design changes were implemented; however, SAC had lost interest in the escort fighter concept. To accompany the B-70 all the way to its target and back, the F-108 in its initial concept would have, at best, marginal range. On 30 December 1958, YF-108A preproduction aircraft on order were reduced from 31 to 20 test aircraft and the first test flight was delayed from February to April 1961.
Cancellation
Even as the XF-108 program was progressing well, there were signs that would ultimately lead to its eventual cancellation. Unconfirmed Soviet bomber threats, the overwhelming trend toward offensive and defensive nuclear missiles in the late 1950s and early 1960s, as well as rising costs, contributed to the termination of the XF-108. The cancellation was announced on 23 September 1959. Despite the extra money and time spent on the Rapier, it was not wholly in vain; the North American A-5 Vigilante supersonic carrier-based nuclear strike bomber developed for the U.S. Navy, which was later modified into a carrier-based reconnaissance aircraft, retained the fuselage/weapon package and systems design of the Rapier. In many ways the Vigilante could be seen as the successful scaled-down application of the Rapier design principles in a Mach 2 supersonic design.
Hughes Aircraft would continue the development of the advanced fire control system and the GAR-9 missile.