The XB-70A Valkyrie is one of the sleekest, good-looking military aircraft ever built. Unfortunately, the aircraft appeared just at a time when military requirements were changing and the manned bomber was being de-emphasized in favor of intercontinental ballistic missiles.

The XB-70A program came out from the Boeing Aircraft Corporation's MX-2145 Project after Boeing along with the Band Corporation conducted studies relating to the type of weapon system required to deliver high-yield special weapons. The study included intercontinental bombers, delivering both gravity bombs and pilot-less parasite bombers; manned bombers, air-refueled by tankers to attend their ranges and cover round-trip intercontinental distances; manned aircraft and drone bomber combinations; and unmanned bombers. During the study, the Air Force requested to further include possible trade-off information on weight for speed, weight for range and speed for range. Boeing managed to present the requested information on 22 January 1954, pointing out the possibilities of a bomber aircraft powered by chemically augmented nuclear power plants. It was B-70.

In May 1946, the Army Air Forces signed a treaty with the Fairchild Engine and Airplane Corporation, conferring on the highly classified ‘Nuclear Energy for the Propulsion of Aircraft’ program a legal right to exist. Boeing then presented data on a chemically augmented, nuclear-powered aircraft. Boeing realized that it is feasible to develop a weapon system of a reasonable size possessing the unlimited range characteristics of nuclear propulsion, plus a high-altitude, supersonic dash capability. At the same time, both the Convair Corporation and Lockheed Aircraft Corporation, under contracts with the Office of Aircraft Nuclear Propulsion, submitted similar data.

In the fall of 1954, the Air Force Council endorsed two independent but simultaneous development programs, one for a nuclear bomber capable of short bursts of supersonic speed; the other, for a subsonic, chemically powered, conventional bomber. The development of a nuclear-powered weapon system needed of performing a strategic mission of 11,000 nautical miles in radius, of which 1,000 miles were to be traveled at speeds in excess of mach 2, at an altitude of more than 60,000 feet. The Air Force Council's announcement was followed the October publication of General Operational Requirement No. 38. calling for an intercontinental bombardment weapon that would replace the B-52 and stay in service during the decade beginning in 1965. On 22 March 1955, the US Air Force put out a second general operational requirement, No. 82, which superseded No. 38. It called for a piloted strategic intercontinental bombardment weapon system that would be capable of carrying a 20,000-pound load of high-yield nuclear weapons, a requirement increased to 25,000 pounds by a September amendment. 

In 1955, the Air Research and Development Command estimated the weapon system's costs through fiscal year 1962 at $2.5 billion. The estimate covered development, test aircraft, and 30 operational bombers, but assumed that a nuclear bomber would also be developed, that a new engine for the chemically powered bomber would be created, and that the price of certain subsystems, earmarked for the B-70, would be borne by the nuclear aircraft program. In early 1955, the Air Force released another general operational requirement, No. 96, for an intercontinental reconnaissance system having similar objectives as the previously established bombardment system, known as Weapon System 110A. In July, same year, the Air Research and Development Command issued a study requirement of General Operational Requirement 96 that validated a reconnaissance version of the B-70. The reconnaissance system was identified as Weapon System 110L. The two systems were combined soon afterward, becoming in the process Weapon System 110A/L.

Several studies of conventional aircraft had shown that no such performance could be obtained with proven design techniques. The Air Force acknowledged that the ability to satisfy its demands, particularly the radius-of-action and speed requirements, would depend on the use of high-energy fuels, new engines, new design techniques, and some other break-through in the state-of-the-art by the operational date of 1963. The Air Force also made sure that the contractors knew that while range and speed trade-offs would be acceptable in order to assure maximum supersonic dash at a practical gross weight, every reduction would have to be minimal. Finally, the new weapon system's configuration would have to allow for the easy addition of state-of-the-art improved subsystems and components, not initially incorporated.

In order to handle the high skin temperatures created by high-speed flight, a large fraction of the nose structure of the aircraft was manufactured out of titanium, a metal which provides good thermal resistance but which is brittle and difficult to machine. Stainless steel honeycomb structures were used for the aircraft’s external skin to reduce cost and to improve the heat dissipation.

In 1961, President Kennedy announced that the XB-70 program was to be reduced to research only, citing high cost (over $700 million per prototype) and vulnerability. The Kennedy administration felt ICBMs were more cost effective because they were less vulnerable and were cheaper operationally. Although two XB-70 prototypes were built, with the first flight in 1964, the program terminated in 1969. The XB-70 had speed, range, and adequate payload, but it was expensive, not suited to low level penetration, and thus did not compete with ICBMs for strategic funds.

Since the XB-70A was no longer considered as an operational aircraft, it had no weapons bay and did not have any capability of delivering weapons. Only a minimal amount of avionics was fitted, just enough so that the aircraft could fly safely. The internal space that would ordinarily have been used for weapons storage and delivery was taken up by test flight instrumentation. On March 3, 1964, the Air Force cancelled the prototype aircraft, citing budgetary considerations. 

The first XB-70A was rolled out at Air Force Plane 42 in Palmdale, California on May 11, 1964. The first flight of the XB-70A took place on September 21, 1964. The flight was nearly four years later that initially planned back in 1958. The plane flew for about an hour, and then landed at Edwards AFB. Aside from problems with the landing gear, the flight went smoothly.

Supersonic flight was achieved for the first time on the third test flight. On March 4, the XB-70A achieved a speed of Mach 1.8, sustaining supersonic speed for over an hour. A flight time of 50 minutes at speeds of over Mach 2 was achieved on the eighth test flight. The second prototype took off on its first flight on July 17, 1965. It differed from the first example by having 5 degrees of wing dihedral. In addition, it had improved hydraulics, better fuel tanks, and an automatic control system for the air intake ramps which replaced the manual system installed on the first prototype. 62-0001 achieved Mach 3 performance at 70,000 feet for the first time on October 14, 1965. On May 19, 1966, the second prototype maintained a speed of Mach 3 for 33 minutes, covering a distance between Utah and California in only 18 minutes.

On June 8, 1966, the second XB-70A crashed after a coalition with an F-104N. The test program continued with the remaining XB-70A. In March of 1967, the plane was transferred to the National Aeronautics and Space Administration, where it participated in an expanded test flight program designed to verify data for a projected supersonic transport.

The XB-70A made its last flight on February 4, 1969, when it was flown to Wright Patterson AFB in Dayton, Ohio and handed over to the USAF Museum. It is now on display there.