Editing Rockwell B-1 Lancer (section)

===Background===
{{Main|North American XB-70 Valkyrie}}

In 1955, the USAF issued requirements for a new bomber combining the payload and range of the [[Boeing B-52 Stratofortress]] with the Mach&nbsp;2 maximum speed of the [[Convair B-58 Hustler]].<ref name= Jenkins_99_p10>Jenkins 1999, p. 10.</ref> In December 1957, the USAF selected [[North American Aviation]]'s [[North American XB-70 Valkyrie|B-70 Valkyrie]] for this role, a six-engine bomber that could cruise at [[Mach number|Mach]]&nbsp;3 at high altitude ({{convert|70000|ft|m|abbr=on|sigfig=2|disp=or}}).<ref name= Jenkins_p12-13>{{harvnb|Jenkins|1999|pp=12–13.}}</ref><ref name=Jenkins_p15-7>{{harvnb|Jenkins|1999|pp=15–17.}}</ref> [[Soviet Union]] [[interceptor aircraft]], the only effective anti-bomber weapon in the 1950s,<ref name="Schwartz p.118"/> were already unable to intercept the high-flying [[Lockheed U-2]];<ref>Rich, Ben and Leo Janos. ''Skunk Works''. Boston: Little, Brown & Company, 1994. {{ISBN|0-316-74300-3}}.</ref> the Valkyrie would fly at similar altitudes, but much higher speeds, and was expected to fly right by the fighters.<ref name="Schwartz p.118"/>

[[File:North American XB-70 in Flight EC68-2131.jpg|thumb|left|The XB-70 Valkyrie was chosen in 1957 to replace the [[Convair B-58 Hustler|Hustler]] but suffered as a result of a switch in doctrine from a high- to a low-altitude flying profile.]]
By the late 1950s, however, anti-aircraft [[surface-to-air missile]]s (SAMs) could threaten high-altitude aircraft,<ref name=Jenkins_1999_p21>{{harvnb|Jenkins|1999|p=21.}}</ref> as demonstrated by the [[1960 U-2 incident|1960 downing]] of [[Gary Powers]]' U-2.<ref>[http://www.keesings.com/search?kssp_selected_tab=article&kssp_a_id=17425n01sov "May 1960&nbsp;– The U-2 Incident.&nbsp;– Soviet and American Statements."] {{Webarchive|url=https://web.archive.org/web/20110723185702/http://www.keesings.com/search?kssp_selected_tab=article&kssp_a_id=17425n01sov |date=23 July 2011 }} ''Keesing's Record of World Events'', Volume 6, 1960.</ref> The USAF [[Strategic Air Command]] (SAC) was aware of these developments and had begun moving its bombers to low-level penetration even before the U-2 incident. This tactic greatly reduces radar detection distances through the use of [[terrain mask#Terrain masking|terrain masking]]; using features of the terrain like hills and valleys, the line-of-sight from the radar to the bomber can be broken, rendering the radar (and human observers) incapable of seeing it.<ref name=Spick_1986_p6-8/> Additionally, radars of the era were subject to "[[clutter (radar)|clutter]]" from stray returns from the ground and other objects, which meant a minimum angle existed above the horizon where they could detect a target. Bombers flying at low altitudes could remain under these angles simply by keeping their distance from the radar sites. This combination of effects made SAMs of the era ineffective against low-flying aircraft.<ref name=Spick_1986_p6-8>{{harvnb|Spick|1986|pp=6–8.}}</ref><ref name="Schwartz p.119"/> The same effects also meant that low-flying aircraft were difficult to detect by higher-flying interceptors, since their radar systems could not readily pick out aircraft against the clutter from ground reflections (lack of [[look-down/shoot-down]] capability).

The switch from high-altitude to low-altitude flight profiles severely affected the B-70, the design of which was tuned for high-altitude performance. Higher [[aerodynamic drag]] at low level limited the B-70 to subsonic speed while dramatically decreasing its range.<ref name=Jenkins_1999_p21/> The result would be an aircraft with somewhat higher subsonic speed than the B-52, but less range. Because of this, and a growing shift to the [[intercontinental ballistic missile]] (ICBM) force, the B-70 bomber program was cancelled in 1961 by President [[John F. Kennedy]],<ref name="Schwartz p.118">{{harvnb|Schwartz|1998|p=118.}}</ref><ref>[https://ntrs.nasa.gov/api/citations/19950002358/downloads/19950002358.pdf "NASA-CR-115702, B-70 Aircraft Study Final Report, Vol. I, p. I-38."] {{Webarchive|url=https://web.archive.org/web/20210412130342/https://ntrs.nasa.gov/api/citations/19950002358/downloads/19950002358.pdf |date=12 April 2021 }} ''NASA'', 1972.</ref> and the two XB-70 prototypes were used in a supersonic research program.<ref name=Jenkins_p14-6>{{harvnb|Jenkins|1999|pp=14–16.}}</ref>

Although never intended for the low-level role, the B-52's flexibility allowed it to outlast its intended successor as the nature of the air war environment changed. The B-52's huge fuel load allowed it to operate at lower altitudes for longer times, and the large airframe allowed the addition of improved [[radar jamming and deception]] suites to deal with radars.<ref>{{harvnb|Knaack|1988|pp=279–280.}}</ref> During the [[Vietnam War]], the concept that all future wars would be nuclear was turned on its head, and the "big belly" modifications increased the B-52's total bomb load to {{convert|60000|lb|kg}},<ref>{{harvnb|Knaack|1988|p=256.}}</ref> turning it into a powerful tactical aircraft which could be used against ground troops along with strategic targets from high altitudes.<ref name="Schwartz p.119"/> The much smaller bomb bay of the B-70 would have made it much less useful in this role.