Although the XB-70 Valkyrie, North American Aviation's Mach 3 bomber, was intended to be immune to enemy attack due to its speed, North American Aviation very briefly proposed the XF-108 Rapier interceptor for the escort role. In this case, the term "penetration fighter" was used, as the aircraft was not expected to actually escort the bombers, and was instead intended to fly into Soviet airspace well in advance of the bombers and attack the Soviet interceptors long before they could approach the bombers.