As an offshoot of Boeing's MX-2145 manned boost-glide bomber project, Boeing partnered with RAND Corporation in January 1954 to explore what sort of bomber aircraft would be needed to deliver the various nuclear weapons then 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. They also concluded that after release of bombs the aircraft would need supersonic speed to escape the critical blast-radius.
The aviation industry had been studying this problem for some time. From the mid-1940s, there was much interest in using nuclear-powered aircraft in the bomber role.[N 1] 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 use during high-power portions of flight—take-off and high-speed dashes.
Another possibility being explored at the time was the use of boron-enriched "zip fuels", which improve the energy density of jet fuel by about 40 percent, and could be used in modified versions of existing jet engine designs. Zip fuels appeared to offer sufficient performance improvement to produce a strategic bomber with supersonic speed.
The U.S. Air Force (USAF) 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.[N 2] The new bomber was expected to enter service in 1963. Both nuclear and conventional designs would be considered. The nuclear-powered bomber was organized as "Weapon System 125A" and pursued simultaneously with the jet-powered version, "Weapon System 110A".
NAA's original proposal for WS-110A. The "floating panels" are large fuel tanks the size of a B-47
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 1,000 nautical miles (1,900 km) entrance and exit from the target. The requirement also called for a 50,000 pounds (23,000 kg) payload and a combat radius of 4,000 nautical miles (4,600 mi; 7,400 km). 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. Boeing and North American Aviation (NAA) submitted proposals, and on 8 November 1955 were awarded contracts for Phase 1 development.
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 featured 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 in spite of its effects on visibility.
The two designs had takeoff weights of approximately 750,000 pounds (340,000 kg) with large fuel loads. The Air Force evaluated the designs, and in September 1956 deemed them too large and complicated for operations. General Curtis LeMay was dismissive, declaiming, "This is not an airplane, it's a three-ship formation." The USAF ended Phase 1 development in October 1956 and instructed the two contractors to continue design studies.
During the period that 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 and widely varying intake ramp air speeds were also under design, allowing for sustained supersonic speeds.
This work led to an interesting discovery. When an engine was optimized specifically for high speed, it burned perhaps twice as much fuel at that speed than when it was running at subsonic speeds. However, the aircraft would be flying as much as four times as fast. Thus its most economical cruise speed, in terms of fuel per mile, was its maximum speed. This was entirely unexpected and implied that there was no point in the dash concept; if the aircraft was able to reach Mach 3, it may as well fly its entire mission at that speed. The question remained whether such a concept was technically feasible, but by March 1957, engine development and wind tunnel testing had progressed enough to suggest it was.
WS-110 was redesigned as an aircraft flying Mach 3 for the entire mission. Zip fuel was retained for the engine's afterburner to increase range. 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, like the Hustler.
NAA's final WS-110A proposal, built as the XB-70
North American had scoured the literature to find any additional advantage. This led them to an obscure report by two NACA wind tunnel experts, who wrote a report in 1956 titled "Aircraft Configurations Developing High Lift-Drag Ratios at High Supersonic Speeds". 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 former individually podded engines were repositioned in a single large duct 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. NAA's solution had an additional advantage, as it decreased the surface area of the rear of the wing when the panels were moved into their high-speed position. This helped offset the natural rearward shift of the center of pressure, or "average lift point", with increasing speeds. Under normal conditions this caused an increasing nose-down trim, which had to be offset by moving the control surfaces, increasing drag. When the wing tips were drooped, the surface area at the rear of the wings was lessened, moving the lift forward and reducing trim drag.
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 450 °F (230 °C), with leading edges reaching 630 °F (330 °C), and up to 1,000 °F (540 °C) 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. For cooling 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 70,000–75,000 ft (21,000–23,000 m), a range of up to 10,500 mi (16,900 km), and a gross weight not to exceed 490,000 pounds (220,000 kg). 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.
In February 1958, the proposed bomber was designated B-70, with the prototypes receiving the "X" experimental prototype designation. The name "Valkyrie" was the winning submission in early 1958, selected from 20,000 entries in a USAF "Name the B-70" contest. The Air Force approved an 18-month program acceleration in March 1958 that rescheduled the first flight to December 1961. But in late 1958 the service announced that this acceleration would not be possible due to lack of funding. 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 planned to use a high-speed, high-altitude bombing approach that followed a trend of bombers flying progressively faster and higher since the start of manned 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. Most forces reached the same conclusion soon after, with both the US and UK starting missile development programs before the war ended. 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.
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 defences. 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 Guideline missile, this minimum altitude was roughly 2,000 feet (610 m). Flying below this would make the bomber effectively invulnerable to the missiles, even if they happened to fly into range.
Flying at low level provided protection against fighters as well. Radars of the era did not have the ability to look down; if the radar were 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, 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 bombload and shorter range. Its only major advantage would be its ability to use high speed in areas without missile cover, especially on the long journey from the USA to USSR. The value was limited; the USAF's doctrine stressed that the primary reason for maintaining the bomber force in an era of ICBMs was that the bombers could remain in the air at long ranges from their bases and were thus immune to sneak attack. 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. After burning, the fuel turned into liquids and solids that increased wear on moving turbine engine components.[N 3] Although the B-70 was intended to use zip only in the afterburners, and thus avoid this problem, the enormous cost of the zip program for such limited gains led to its cancellation. This by itself was not a fatal problem, however, as newly developed high-energy fuels like JP-6 were available to make up some of the difference. Most of the range lost in the change from zip fuel was restored by filling one of the two bomb bays with a fuel tank. However, another problem arose when the F-108 program was canceled in September 1959, which ended the shared development that benefited the B-70 program.
Downsizing, upswing, cancellation
At two secret meetings on 16 and 18 November 1959, the Chairman of the Joint Chiefs of Staff, Air Force General Twining, recommended the Air Force's plan for the B-70 to reconnoiter and strike rail-mobile Soviet ICBMs, but the Chief of Staff of the Air Force, General White, admitted the Soviets would "be able to hit the B-70 with rockets" and requested the B-70 be downgraded to "a bare minimum research and development program" at $200 million for fiscal year 1960. President Eisenhower responded that the reconnaissance and strike mission was "crazy" since the nuclear mission was to attack known production and military complexes, and emphasized that he saw no need for the B-70 since the ICBM is "a cheaper, more effective way of doing the same thing". Eisenhower also identified that the B-70 would not be in manufacturing until "eight to ten years from now" and "said he thought we were talking about bows and arrows at a time of gunpowder when we spoke of bombers in the missile age". 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.
XB-70A parked at Edwards Air Force Base in 1967
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 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 for the B-70 development program.
On taking office in January 1961, Kennedy was informed that the missile gap was an illusion.[N 4] On 28 March 1961, after $800 million 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 Congress approved $290 million of B-70 "add-on" funds to the President's 12 May 1960 modified FY 1961 budget, the Administration decided on a "Planned Usage" of only $100 million of these funds. The Department of Defense subsequently presented data to Congress that the B-70 would add little performance for the high cost.
However, 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 in an effort 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—with 21 members having 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 appropriated for the RS-70. 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.
XB-70A on the taxiway on 21 September 1964, the day of the first flight
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 7 May 1964, and rolled out on 11 May 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 first XB-70 carried out its maiden flight in September 1964 and many more test flights followed.
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.[N 5][N 6] 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.