Background In an offshoot of
Boeing's MX-2145 crewed
boost-glide bomber project, Boeing hired
RAND Corporation in January 1954 to explore what sort of bomber aircraft would be needed to deliver the various
nuclear weapons under development. At the time, nuclear weapons weighed several tons, and the need to carry enough fuel to fly that payload from the
continental United States to the
Soviet Union demanded large bombers. Boeing and RAND also concluded that the aircraft would need supersonic speed to escape the blast of its bombs. The aviation industry had been studying this problem for some time. From the mid-1940s, there was interest in using
nuclear-powered aircraft as bombers. In a conventional jet engine, thrust is provided by heating air using
jet fuel and accelerating it out a nozzle. In a nuclear engine, heat is supplied by a reactor, whose consumables last for months instead of hours. Most designs also carried a small amount of jet fuel for high-power portions of flight, such as takeoffs and high-speed dashes. Zip fuels appeared to offer sufficient performance improvement to produce a strategic bomber with supersonic speed.
WS-110A The Air Force followed these developments closely, and in 1955 issued General Operational Requirement No. 38 for a new bomber, combining the payload and intercontinental range of the
B-52 with the Mach 2 top speed of the
Convair B-58 Hustler. The new bomber was expected to enter service in 1963. Nuclear and conventional designs were considered. The nuclear-powered bomber was organized as "
Weapon System 125A" and pursued simultaneously with the jet-powered version, "Weapon System 110A". . Boeing's design was almost identical, differing largely in having a single vertical stabilizer and having two of its engines in pods at the outer edges of the inner wing section. The USAF Air Research and Development Command's (ARDC) requirement for WS-110A asked for a chemical-fuel bomber with Mach 0.9 cruising speed and "maximum possible" speed during a entrance and exit from the target. The requirement also called for a payload and a combat radius of . The Air Force formed similar requirements for a WS-110L intercontinental reconnaissance system in 1955, but this was later canceled in 1958 due to better options. In July 1955, six contractors were selected to bid on WS-110A studies. In mid-1956, initial designs were presented by the two companies. Zip fuel was to be used in the
afterburners to improve range by 10 to 15 percent over conventional fuel. Both designs had huge wing-tip fuel tanks that could be jettisoned when their fuel was depleted before a supersonic dash to the target. The tanks also included the outer portions of the wing, which would also be jettisoned to produce a smaller wing suitable for supersonic speeds. Both became
trapezoidal wings after ejection, at that time the highest performance
planform known. They also featured flush
cockpits to maintain the highest
fineness ratio possible despite its negative effects on visibility. The USAF ended Phase 1 development in October 1956 and instructed the two contractors to continue design studies.
New designs While the original proposals were being studied, advances in supersonic flight were proceeding rapidly. The narrow delta was establishing itself as a preferred planform for supersonic flight, replacing earlier designs like the swept-wing and trapezoidal layouts seen on designs like the
Lockheed F-104 Starfighter and the earlier WS-110 concepts. Engines able to cope with higher temperatures were also under development, allowing for sustained supersonic speeds. Both North American and Boeing returned new designs with very long fuselages and large delta wings. They differed primarily in engine layout; the NAA design arranged its six engines in a semi-circular duct under the rear fuselage, while the Boeing design used separate podded engines located individually on pylons below the wing, Known today as
compression lift, the idea was to use the
shock wave generated off the nose or other sharp points on the aircraft as a source of high-pressure air. By carefully positioning the wing in relation to the shock, the shock's high pressure could be captured on the bottom of the wing and generate additional lift. To take maximum advantage of this effect, they redesigned the underside of the aircraft to feature a large triangular intake area far forward of the engines, better positioning the shock in relation to the wing. The six individually-podded engines were repositioned, three in each of two separate ducts, under the fuselage. North American improved on the basic concept by adding a set of drooping wing-tip panels that were lowered at high speed. This helped trap the shock wave under the wing between the downturned wing tips. It also added more vertical surface to the aircraft to maintain directional stability at high speeds. The buildup of heat due to
skin friction during sustained
supersonic flight had to be addressed. During a Mach 3 cruise, the aircraft would reach an average of , with
leading edges reaching , and up to in engine compartments. NAA proposed building their design out of
sandwich panels, with each panel consisting of two thin sheets of stainless steel brazed to opposite faces of a
honeycomb-shaped foil core. Expensive
titanium would be used only in high-temperature areas like the leading edge of the horizontal stabilizer, and the nose. To cool the interior, the XB-70 pumped fuel en route to the engines through heat exchangers. On 30 August 1957, the Air Force decided that enough data were available on the NAA and Boeing designs that a competition could begin. On 18 September, the Air Force issued operational requirements that called for a cruising speed of Mach 3.0 to 3.2, an over-target altitude of , a range of up to , and a gross weight not to exceed . The aircraft would have to use the hangars, runways and handling procedures used by the B-52. On 23 December 1957, the North American proposal was declared the winner of the competition, and on 24 January 1958, a contract was issued for Phase 1 development. The Air Force approved an 18-month program acceleration in March 1958 that rescheduled the first flight to December 1961. In December 1958, a Phase II contract was issued. The mockup of the B-70 was reviewed by the Air Force in March 1959. Provisions for air-to-surface missiles and external fuel tanks were requested afterward. At the same time, North American was developing the
F-108 supersonic interceptor. To reduce program costs, the F-108 would share two of the engines, the escape capsule, and some smaller systems with the B-70. In early 1960, North American and the USAF released the first drawing of the XB-70 to the public.
The "missile problem" The B-70 was designed to use a high-speed, high-altitude bombing approach that followed a trend of bombers flying progressively faster and higher since the start of crewed bomber use. Through that same period, only two weapons proved effective against bombers:
fighter aircraft and
anti-aircraft artillery (AAA). Flying higher and faster made it more difficult for both; higher speeds allowed the bomber to fly out of range of the weapons more quickly, while higher altitudes increased the time needed for fighters to climb to the bombers, and greatly increased the size of the AAA weapons needed to reach those altitudes. As early as 1942, German flak commanders had already concluded that AAA would be essentially useless against jet aircraft, and began development of guided missiles to fill this role. The UK's
Green Mace was one of the last attempts to develop a useful high-altitude AAA weapon, but its development ended in 1957.
Interceptor aircraft with ever-improving performance remained the only effective anti-bomber weapons by the early 1950s, and even these were having problems keeping up with the latest designs; Soviet interceptors during the late 1950s could not intercept the high-altitude
U-2 reconnaissance aircraft, despite its relatively low speeds. It was later discovered that flying faster also made radar detection much more difficult due to an effect known as the
blip-to-scan ratio, and any reduction in tracking efficiency would further interfere with the operation and guidance of fighters. The introduction of the first effective anti-aircraft missiles by the late 1950s changed this picture dramatically. Missiles could stand ready for immediate launch, eliminating operational delays like the time needed to get the pilot into the cockpit of a fighter. Guidance did not require wide-area tracking or calculation of an intercept course: a simple comparison of the time needed to fly to the altitude of the target returned the required
deflection. Missiles also had greater altitude capability than any aircraft and improving this to adapt to new aircraft was a low-cost development path. The US was aware of Soviet work in the field, and had reduced the expected operational lifetime of the U-2, knowing that it would become vulnerable to these missiles as they were improved. In 1960, a U-2 flown by
Gary Powers was
shot down by one of the earliest Soviet guided air-defence missiles, the
S-75 Dvina, known in the west as the SA-2 Guideline. Faced with this problem, military doctrine had already started shifting away from high-altitude supersonic bombing toward low-altitude
penetration. Radar is line-of-sight, so aircraft could dramatically shorten detection distances by flying close to the Earth and hiding behind terrain. Missile sites spaced to overlap in range when attacking bombers at high altitudes would leave large gaps between their coverage for bombers flying at lower levels. With an appropriate map of the missile sites, the bombers could fly between and around the defenses. Additionally, early missiles generally flew unguided for a period of time before the radar systems were able to track the missile and start sending it guidance signals. With the SA-2 missile, this minimum altitude was roughly . Flying at low level provided protection against fighters as well. Radars of the era did not have the ability to look down (see
look-down/shoot-down); if a higher altitude aircraft's radar was aimed down to detect targets at a lower altitude, the reflection of the ground would overwhelm the signal returned from a target. An interceptor flying at normal altitudes would be effectively blind to bombers far below it. The interceptor could descend to lower altitudes to increase the amount of visible sky, but doing so would limit its radar range in the same way as the missile sites, as well as greatly increasing fuel use and thus reducing mission time. The Soviet Union would not introduce an interceptor with look-down capability until 1972 with the High Lark radar in the
MiG-23M, and even this model had very limited capability. Strategic Air Command found itself in an uncomfortable position; bombers had been tuned for efficiency at high speeds and altitudes, performance that had been purchased at great cost in both engineering and financial terms. Before the B-70 was to replace the B-52 in the long-range role, SAC had introduced the
B-58 Hustler to replace the
Boeing B-47 Stratojet in the medium-range role. The Hustler was expensive to develop and purchase, and required enormous amounts of fuel and maintenance in comparison to the B-47. It was estimated that it cost three times as much to operate as the much larger and longer-ranged B-52. The B-70, designed for even higher speeds, altitudes and range than the B-58, suffered even more in relative terms. At high altitudes, the B-70 was as much as four times as fast as the B-52, but at low altitudes it was limited to only Mach 0.95, only modestly faster than the B-52 at the same altitudes. It also had a smaller bomb load and shorter range. In this case, the higher speed would be used for only a short period of time between the staging areas and the Soviet coastline. Adding to the problems, the zip fuel program was canceled in 1959. Another problem arose when the
XF-108 program was canceled in September 1959, which ended the shared development that benefited the B-70 program. In December 1959 the Air Force announced the B-70 project would be cut to a single prototype, and most of the planned B-70 subsystems would no longer be developed. Then interest increased due to the politics of
presidential campaign of 1960. A central plank of
John F. Kennedy's campaign was that Eisenhower and the Republicans were weak on defense, and pointed to the B-70 as an example. He told a San Diego audience near NAA facilities, "I endorse wholeheartedly the B-70 manned aircraft." Kennedy also made similar campaign claims regarding other aircraft: near the Seattle Boeing plant he affirmed the need for B-52s and in Fort Worth he praised the B-58. The Air Force changed the program to full weapon development and awarded a contract for an XB-70 prototype and 11 YB-70s in August 1960. In November 1960, the B-70 program received a $265 million (equivalent to $ billion today) appropriation from Congress for FY 1961. Nixon, trailing in his home state of California, also publicly endorsed the B-70, and on 30 October Eisenhower helped the Republican campaign with a pledge of an additional $155 million ($ billion today) for the B-70 development program. On 28 March 1961, after $800 million (equivalent to $ billion today) had been spent on the B-70 program, Kennedy canceled the project as "unnecessary and economically unjustifiable" because it "stood little chance of penetrating enemy defenses successfully." Instead, Kennedy recommended "the B-70 program be carried forward essentially to explore the problem of flying at three times the speed of sound with an airframe potentially useful as a bomber." After becoming the new Air Force Chief of Staff in July 1961,
Curtis LeMay increased his B-70 advocacy, including interviews for August ''Reader's Digest
and November Aviation Week'' articles, and allowing a 25 February
General Electric tour at which the press was provided artist conceptions of, and other info about, the B-70. Congress had also continued B-70 appropriations to resurrect bomber development. After Secretary of Defense
Robert McNamara explained again to the
House Armed Services Committee (HASC) on 24 January 1962 that the B-70 was unjustifiable, LeMay subsequently argued for the B-70 to both the House and Senate committees—and was chastised by McNamara on 1 March. By 7 March 1962, the HASC, 21 of whose members had B-70 work in their districts, had written an appropriations bill to "direct"—by law—the Executive Branch to use all of the nearly $500 million (equivalent to $ billion today) appropriated for the RS-70 (see
Variants). McNamara was unsuccessful with an address to the HASC on 14 March, but a 19 March 1962
11th hour White House Rose Garden agreement between Kennedy and HASC chairman
Carl Vinson retracted the bill's language and the bomber remained canceled.
Experimental aircraft The XB-70s were intended to be used for the advanced study of
aerodynamics,
propulsion, and other subjects related to large supersonic transports. The crew was reduced to only the two pilots, as a
navigator and a
bombardier were not needed for this research role. The production order was reduced to three prototypes in March 1961 with the third aircraft to incorporate improvements from the previous prototype. The order was later reduced to two experimental XB-70As, named Air Vehicle 1 and 2 (AV-1 and AV-2). XB-70 No. 1 was completed on 7May 1964, and rolled out on 11May 1964 at
Palmdale, California. One report stated "nothing like it existed anywhere". AV-2 was completed on 15 October 1964. The manufacture of the third prototype (AV-3) was canceled in July 1964 before completion. The data from the XB-70 test flights and aerospace materials development were used in the later
B-1 bomber program, the American
supersonic transport (SST) program, and via espionage, the
Soviet Union's
Tupolev Tu-144 SST program. The development of the
Lockheed U-2 and the
SR-71 Blackbird reconnaissance aircraft, as well as the XB-70, prompted Soviet
aerospace engineers to design and develop their high-altitude and high-speed
MiG-25 interceptor. == Design ==