However, by mid-1959, the Air Force was already beginning to experience some doubts about the high cost of the Rapier program. The primary strategic threat from the Soviet Union was now perceived to be its battery of intercontinental ballistic missiles instead of its force of long-range bombers. Against intercontinental ballistic missiles, the F-108A interceptor would be completely useless. In addition, the Air Force was increasingly of the opinion that unmanned intercontinental ballistic missiles could accomplish the mission of the B-70 Valkyrie/F-108 Rapier combination much more effectively and at far lower cost. Consequently, the F-108A project was cancelled in its entirety on 23 September 1959, before any prototypes could be built.