In the decades following World War 2 (1939-1945) there stood a period of immense achievements related to supersonic flight. The first jet-versus-jet fighter duels occurred over Korea in the Korean War (1950-1953) and an all-new generation of jet-powered heavy bombers emerged to replace the prop-powered classics like the Boeing B-29/B-50 "Superfortress". The quintessential United States Air Force (USAF) heavy bomber then became the eight-engine Boeing B-52 "Stratofortress" which brought along with it a very-high-altitude (50,000 feet) bombing capability. To counter this threat, the Soviets adopted a progressive series of interceptors until its missile defense network ultimately became the real danger for American bomber crews.
North American Aviation
North American Aviation - founded in 1928 and primarily recognized known for its development of the war-winning P-51 "Mustang" in World War 2 and the F-86 Sabre in the Korean War - still remained a prominent defense player during these years and continued to produce notable aircraft like the FJ "Fury" series, the F-100 "Super Sabre", and the record-setting X-15 rocket research plane. By the end of the 1950s, its A-5 "Vigilante" would be adopted b y the United States Navy (USN) and see combat service in the Vietnam War (1955-1975).
The High-Altitude Bomber Need
A key developmental product for the company during the latter part of the 1950s became the XB-70 "Valkyrie", a supersonic strategic bomber that was to use Mach 3+ speeds to penetrate enemy airspace while carrying a nuclear war load and exit the danger area before the enemy could appropriately react. The aircraft was intended to replace the venerable B-52 in the high-flying bomb delivery role and serve with United States Strategic Air Command (SAC) as part of the North American/European defense against an all-out Soviet nuclear attack. The primary quality of the new aircraft was to be its inherent speed which would theoretically allow it to outrun any intercepting aerial foe or ground-launched threats. However, advances in missile technology by the Soviets ultimately ruled such aircraft moot into the 1960s - this exemplified by the shooting down of a high-altitude Lockheed U-2 over the Soviet Union in the "Gary Powers Incident" of 1960.
However, before missile technology went on to rule war planner's dreams, the high-altitude bomber concept was still a viable battlefield tool. Defense companies continued work into various nuclear delivery methods during this time - a time when the United States nuclear defense network centered on a "triad" approach in which aircraft (strategic bombers), submarines (SLMBs = "Submarine-Launched Ballistic Missiles"), and land-based launchers (ICBMs = "InterContinental Ballistic Missiles") worked in conjunction to delivery nuclear ballistic missiles against the enemy. Should one corner of the triad fail from an initial ("first-strike") Soviet attack, the other two acted as a failsafe to ensure the destruction of the enemy in turn.
During the 1940s, the concept of a nuclear-powered bomber was explored but these designs eventually fell to naught as overly complicated ventures and placed the focus back on traditionally-powered jet bombers. Advances were also still ongoing in the realm of supersonic flight which further pushed the idea of an ultra-fast bomber into perspective and from this requirement both Boeing and North American led the way with internal designs centering on a dimensionally large, multi-engine layout with a delta wing planform. With help from the NAA - forerunner to today's NASA - the North American product moved ahead to test several of its concept as sound.
The USAF and the Valkyrie
During August 1957, the USAF pushed ahead with a direct competition between Boeing and North American. The basic requirements for the new USAF bomber included a cruising speed of Mach 3 with a flying altitude of over 70,000 feet. Range was a key quality as the bomber would be required to traverse long distances so this value centered around 10,500 miles. Structurally, the aircraft would be no larger than the B-52 already in service so as to reuse existing facilities for the new product. Gross weight of the aircraft would be restricted to 490,000 lb. A crew of four was envisioned encompassing pilot, co-pilot, navigator, and bombardier. The North American submission was the one that best filled the USAF requirement and was selected the winner in December of that year with a contract announced in January. From another contest held in early 1958 to name the aircraft, the winning entry of "Valkyrie" was selected. It received the formal development designation by the USAF of "XB-70".
Work began at a what was a quickened pace but budget issues slowed the program throughout 1958 and 1959. In March of the latter, the USAF was allowed a review of an XB-70 mockup aircraft which, of course, unveiled some new requirements as well as alterations by the branch. In 1960, the design was officially unveiled to the American public (though in drawn form).
The XB-70 was a technological marvel of the 1950s. In its originally conceived bomber form it was to carried a full crew of four as mentioned while the aircraft showcased one of the most identifiable shapes of any aircraft in history with its tubular fuselage, large delta wing planform, and underslung engines. The cockpit was held in its traditional position at the front of the fuselage with large forward canards straddling the crew area section. The engines numbered six in a horizontal arrangement and these were aspirated by two large angular intake openings under the aircraft. The wing mainplanes were positioned under the fuselage though above the engines and applicable intake ductwork and the like. Two vertical fins were seated outboard of the six engine exhaust ports at rear. The wing mainplanes also held wing tips that could vary their angles in-flight - angled downwards up to 65-degrees for increased stability at supersonic speeds. The undercarriage was made up of two main legs with four wheels each and a two-wheeled nose leg. All of the legs were seated within the lower structure of the aircraft.
The engine of choice for the XB-70 became the General Electric YJ93-GE-3 turbojet which offered an afterburning capability (at the expense of higher fuel consumption). Dry thrust output reached 19,900lbf (each engine) with afterburning thrust reaching 28,800lbf (each engine). All told, performance included a maximum speed of 2,056 mph (up to Mach 3.1), a cruise speed of 2,000 mph (Mach 3.0), a service ceiling of 77,250 feet, and a range out to 4,288 miles with a combat load in tow.
Body construction of the Valkyrie involved a "honeycombed" core encased in stainless steel panels (sandwich arrangement) while titanium was used at high temperature surfaces (much heat was generated as the speeds/altitudes required of the aircraft). The overall design of the aircraft was made to take advantage of "compression lift", a naturally occurring byproduct of flight appearing at high speed/high altitude that could use some of the generated shock waves as high pressure air to generate additional lift. This approach gave the XB-70 a wholly unique appearance in relation to other aircraft of the period (its sharp angles, the intake positioning, etc...). The lowering of the wingtips was another high-speed design feature meant to aid directional stability during flying.
The XB-70 was to have individual encapsulated seats for ejection in the case of emergency. A whole-cabin ejection system was also tested.
The Evolution of Missile Technology
Missile defenses were in use since the 1950s and in the 1960s, the available technology was such that it began rendering such high-flying supersonic bomber products like the XB-70 obsolete. This led to a late-November 1959 request by authorities to revise the XB-70 as strike-reconnaissance platform to target Soviet missile launchers. However, this initiative was not followed and the XB-70 program suffered a reduced development role instead as its initial value quickly diminished. The rise of America's own ICBM missile program also helped to derail the XB-70's development further. In late 1959, the XB-70 program's funding covered just one prototype.
Popular support for the XB-70 then began to rise due to public mentions during the 1960 presidential campaign. The XB-70 gained new life to the point that the USAF commissioned for completion of an XB-70 prototype and this to be followed by eleven improved developmental vehicles under the "YB-70" designation. Funding followed in November of 1960.
With John F. Kennedy's victory over Richard Nixon, Kennedy's new administration moved to nix the XB-70 bomber program in full in March of 1961 and shift American priorities to missiles. It was decided to retain the services of the XB-70 as a research and development product exploring Mach 3 flight. A short-lived initiative to revive the XB-70 as a missile-carrying reconnaissance-strike aircraft (as the "RS-70") was not enough to resurrect the XB-70 bomber program. This revised version was to keep its crew of four and sport an in-flight refueling capability. Regardless, the reduced research role of the XB-70 would continue and modifications to the original design were enacted to better serve the new purpose.
The XB-70 as a Research Platform
Despite three XB-70s planned for the new program, the procurement was later reduced to two airframes under the "XB-70A" designation. The aircraft were individually recognized as "AV-1" and "AV-2" (AV = "Air Vehicle)". AV-1 was arrived in May of 1964 and AV-2 followed in October. First flight of AV-1 took place on September 21st, 1964 though this event was marred by one engine shutdown and an undercarriage warning that forced the crew to fly the large aircraft with its legs down. Furthermore, upon landing, the rear wheels along the portside of the aircraft locked in place and this friction then led to their rubber rupturing and a flame starting - an ominous start for the long-gestating program.
Mach 1 flight for the XB-70 was not experienced until October of 1964 and it was not until October of 1965 that Mach 3 flight (at 70,000 feet) was finally attained. Improvements discovered through testing of the AV-1 were instituted into the AV-2 which recorded its own first flight on July 17th, 1965. Further work evolved AV-2 to serve in sonic boom testing with the aircraft appropriately outfitted with special components and systems.
Tragedy Strikes the Program
On June 8th, 1966, AV-2 was used in a General Electric photograph promotional (for a corporate brochure) which attempted to showcase all of the then-current USAF aircraft outfitted with GE engines in one shot (formation flying involving four total aircraft). However, the formation was rocked when the accompanying Lockheed F-104 "Starfighter" was pulled into a vortex air stream of the XB-70, sucking the F-104 into the XB-70's wing and driving into the large aircraft's vertical stabilizers (fins). The F-104 erupted into flames while the XB-70 - initially continuing in level flight for some 16 seconds, its crew unaware of the contact - became unstable, began an uncontrollable (flat) spin, and entered into a downward flight path. The incident further stained the career of the expensive bomber program that began which so much promise in the late 1950s. Of the three pilots involved - Al White and Joe Cross in the XB-70 and Joe Walker in the F-104 - only White survived after having used his escape capsule against the growing G-forces of the massive aircraft.
General Electric certainly received publicity from the accident - though not the kind it sought.
The End of the Road
AV-1 continued on as a test vehicle into 1969. After 83 total flights (the last to Wright-Patterson AFB), the airframe was officially retired to become a protected showpiece of the National Museum of the United States Air Force in Dayton, Ohio.
While a failed bomber project on its own, the data collected from the Valkyrie as a research platform became priceless for new generation bomber projects and supersonic flight in general. Indeed, results were used in the development of the low-altitude Rockwell B-1 "Lancer" penetration bomber of USAF/SAC. The Mach 3-capable XB-70 also forced the Soviet Union to develop their Mach 2.5-capable Mikoyan-Gurevich MiG-25 "Foxbat" as an XB-70 counter. While the B-70 failed to see the light of day in service, the Foxbat has gone on to have a lengthy flying career for the Russians and others. The Tupolev Tu-144 supersonic passenger airliner is also said to have benefitted from stolen XB-70 data - it certainly holds some design features of the XB-70 prototype as well as that of the European Aerospatiale / BAC Concorde airliner. Another North American project - the abandoned XF-108 "Rapier" supersonic interceptor - was also to have used the GE engine (two side-by-side) developed for the XB-70.
The XB-70 also failed as the successor to the storied B-52 and the Boeing product also survived the B-1 Lancer as its other designated replacement. Due to the procurement and operating costs of the B-1 bomber and the Northrop B-2 "Spirit" stealth bomber, the B-52 has remained in service with the USAF - all three operated concurrently.