Designed in the late 1950s by North American Aviation (NAA) to replace the B-52 Stratofortress and B-58 Hustler,[1] the six-engine, delta-winged Valkyrie[2] could cruise for thousands of miles at Mach 3+ while flying at 70,000 feet (21,000 m).

The bomber would spend only a brief time over a particular radar station, flying out of its range before the controllers could position their fighters in a suitable location for an interception.

In response, the US Air Force (USAF) began flying its missions at low level, where the missile radar's line of sight was limited by terrain.

In 1966, one prototype crashed after colliding with an F-104 Starfighter while flying in close formation; the remaining Valkyrie bomber is in the National Museum of the United States Air Force near Dayton, Ohio.

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.

[22] 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.

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.

[24] North American improved on the basic concept by adding a set of drooping wing-tip panels that were lowered at high speed.

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 miles (16,900 km), and a gross weight not to exceed 490,000 pounds (220,000 kg).

[34] 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.

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.

[43] 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.

[6] After burning, the fuel turned into caustic and abrasive liquids and solids that increased wear on moving turbine engine components and were toxic, making servicing difficult.

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".

"[50] 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.

[53][54] 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 ($1.6 billion today) for the B-70 development program.

[56][N 4] On 28 March 1961,[57] after $800 million (equivalent to $8.2 billion today) had been spent on the B-70 program, Kennedy canceled the project as "unnecessary and economically unjustifiable"[55] because it "stood little chance of penetrating enemy defenses successfully.

"[58] 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.

[59] 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.

Harrison Storms shaped the aircraft[75] with a canard surface and a delta wing, which was built largely of stainless steel, sandwiched honeycomb panels, and titanium.

[76] The XB-70 used compression lift, which resulted from a shock wave generated by the leading edge of the engine intake splitter below the apex of the wing.

[78] Camber was added to the wing leading edge inboard of the folding tips to improve subsonic handling and reduce supersonic drag.

[79] Like a number of other delta-wing aircraft designed for supersonic speeds (Concorde, Tu-144, FD2), the Valkyrie needed a feature to improve the pilot's view during nose-high low-speed flight and on the ground.

Testing was planned to cover a range of sonic boom overpressures on the ground similar to but higher than those anticipated from the proposed American SST.

[100] NASA testing from June 1968 included two small vanes on the nose of AV-1 for measuring the response of the aircraft's stability augmentation system.

[112] On 14 October 1965, AV-1 surpassed Mach 3, but heat and stress damaged the honeycomb panels, leaving 2 ft (60 cm) of the leading edge of the left wing missing.

[1][113] After the photoshoot, the F-104 drifted into the XB-70's right wingtip, flipped and rolled inverted over the top of the Valkyrie, before striking the bomber's vertical stabilizers and left wing.

Despite the loss of both vertical stabilizers and damage to the wings, the Valkyrie flew straight for 16 seconds before it entered an uncontrollable spin and crashed north of Barstow, California.

[114][115] The USAF summary report of the accident investigation stated that, given the position of the F-104 relative to the XB-70, Walker, the F-104 pilot, would not have been able to see the XB-70's wing, except by uncomfortably looking back over his left shoulder.

The report concluded that from that position, without appropriate sight cues, Walker was unable to properly perceive his motion relative to the Valkyrie, leading to his aircraft drifting into the XB-70's wing.