The Rockwell (now part of Boeing) B-1 Lancer is a strategic bomber used by the United States Air Force. First envisioned in the 1960s as a supersonic bomber with sufficient range and payload to replace the B-52 Stratofortress, it developed primarily into a low-level penetrator with long range and supersonic speed capability. Its development was stopped and restarted multiple times over its history, as the theory of strategic balance changed from flexible response to mutually assured destruction and back again. It eventually entered service more than 20 years after first being studied.
The B-1B production version has been in service with the United States Air Force (USAF) since 1986. The Lancer serves as the supersonic component of the USAF's long-range bomber force, along with the subsonic B-52 and B-2 Spirit. The bomber is commonly called the "Bone" (originally from "B-One"). With the retirement of the EF-111 Raven in 1998 and the F-14 Tomcat in 2006, the B-1B is the U.S. military's only active variable-sweep wing aircraft.
In December 1957, U.S. Air Force selected North American Aviation's proposal to replace the B-52 Stratofortress. This would lead to the B-70 Valkyrie. The Valkyrie was a six-engine bomber that could fly very high at Mach 3 to avoid interceptor aircraft, the only effective anti-bomber weapon in the 1950s. At the time, Soviet interceptors were unable to intercept the high-flying Lockheed U-2; the Valkyrie was to fly at similar altitudes and much higher speeds. But by the late 1950s, anti-aircraft surface-to-air missiles (SAMs) could threaten high-altitude aircraft, as demonstrated by the 1960 downing of Gary Powers' U-2.
Recognizing this, the USAF Strategic Air Command had begun moving to low-level penetration before the U-2 downing. This greatly reduces radar detection distances while at that time SAMs were ineffective against low-flying aircraft, while interceptors did not have as large of a speed advantage at low-level. Also the flight path to a target could be selected so the landscape shields the bomber from the radar's line-of-sight operation. Low-altitude flight also made the bombers difficult to detect from aircraft at higher altitudes, as radar systems of that generation could not "look down" due to the clutter that resulted from ground reflections.