"In June 1953, North American initiated an in-house study of advanced F-100 designs, leading to proposed interceptor (NAA 211: F-100BI denoting "interceptor") and fighter-bomber (NAA 212: F-100B) variants. Concentrating on the F-100B, the preliminary engineering and design work focused on a tactical fighter-bomber configuration, featuring a recessed weapons bay under the fuselage and provision for six hardpoints underneath the wings. Single-point refuelling capability was provided while a retractable tailskid was installed. An all-moving vertical fin and an automated flight control system was incorporated which permitted the aircraft to roll at supersonic speeds using spoilers. The flight control system was upgraded by the addition of pitch and yaw dampers.
The aircraft's most distinguishing feature is its dorsal-mounted variable-area inlet duct (VAID). While the VAID was at the time a system unique to the F-107A, it is now considered to be an early form of variable geometry intake ramp which automatically controlled the amount of air fed to the jet engine. Although the preliminary design of the air intake was originally located in a chin position under the fuselage (an arrangement later adopted for the F-16), the air intake was eventually mounted in an unconventional position directly above and just behind the cockpit.The VAID system proved to be very efficient and NAA used the design concept on their A-5 Vigilante, XB-70 Valkyrie and XF-108 Rapier designs.
The air intake was in the unusual dorsal location as the USAF had required the carriage of an underbelly semi-conformal nuclear weapon. The original chin intake caused a shock wave that interfered in launching this weapon. The implications this had for the survivability of the pilot during ejection were troubling. The intake also severely limited rear visibility. Nonetheless this was not considered terribly important for a tactical fighter-bomber aircraft, and furthermore it was assumed at the time that air combat would be via guided missile exchanges outside visual range.
In August 1954, a contract was signed for three prototypes along with a pre-production order for six additional airframes."