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 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 cancelled in 1959, due to a shortage of funds and the Soviets' adoption of ballistic missiles as their primary means of nuclear attack. 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. The specification was laid down on 6 October 1955, calling for an interceptor that could fly at at a speed of Mach 1.7 (, with a range of . It was to have a two-man crew and at least two engines. A further consideration was that an integrated fire-control system would be fitted, allowing the interception of a bomber at and three targets to be destroyed during a single mission. 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. Of the paper designs, the North American proposal, dubbed "NA-236", seemed the most promising. The NA-236 shared some similarities with the XF-108, although the most obvious differences were the additions of two finlets at the midspan of the horizontal stabilizers, and canards. 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).