The North American product (Model NA-257) began a lengthy design process which moved forward as requirements changed and technology advanced. Hughes was charged with development of the FCS to manage the missiles and General Electric supplied the engines. During this time, North American was also committed to the USAF's XB-70 "Valkyrie" Mach 3 supersonic bomber and, thusly, the XF-108 shared the XB-70s GE engine. The same ejection capsule of the XB-70 was also instituted in the XF-108. The XF-108's design lines also mimicked that of the XB-70 to an extent - the delta wing planform, canard foreplanes, the squared-off underside, etc...
Factors began to work against the XF-108 program - the Soviet commitment to effective missile defense networks (which rendered high-altitude supersonic bombers obsolete) and priority given to InterContinental Ballistic Missiles (ICBMs) for nuclear warhead delivery (nixing the strategic bomber approach) forced USAF interest in a new, expensive interceptor to wane. Early on, the XF-108 was to feature canard foreplanes for pitch control, a base delta wing planform, and no fewer than three vertical stabilizers (one on the fuselage and the other two at each of wing trailing edges). The finalized form was something of a rewriting of the aircraft - showcasing the continual evolution of this interceptor - which saw the canards and two of the three vertical stabilizers (upper sections only) removed while the delta wings became a "double-delta" arrangement (the main wing leading edge swept at 65-degrees and the wingtips with a 45-degree sweep. A full-scale representation of the XF-108 was presented for USAF review in January of 1959. Thirty-one developmental "YF-107" aircraft previously on order were by this time reduced to just 20 aircraft.
The XF-108 program was to produce nothing more than the aforementioned mockup for, on September 23rd, 1959, the USAF cancelled the interceptor. Some of the work put into the XF-108 was reconstituted for the more successful A-5 "Vigilante" reconnaissance-strike bomber by North American which was adopted by the United States Navy (USN) in 1961. The A-5 shared some of the design form of the XF-108 (retaining its fuselage and systems) and saw production reach 156 total units. Vigilante aircraft recorded combat service in the Vietnam War (1955-1975).
Performance specifications of the XF-108 included a maximum speed of Mach 3 and an operational range of 1,150 miles. Weight (gross) was 102,000 lb. 4 x 20mm cannons was to be standard armament while support for 2.75" rockets was to be included as was capability for carrying up to 4,000 lb of stores into action.
The program cost reached $142 million USD. During its early design phase, the XF-108 was also considered for the bomber escort role - escorting the B-70 (XB-70) Valkyrie into enemy airspace. This role was dropped as the XF-108 would have lacked the required range to see the B-70 all the way to and from its target area.
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