The XB-70A Valkyrie
one of the most beautiful aircraft
in history. Its sweeping
lines and gracefully curved fuselage
remind the viewer favorably of the determined stance
of a trumpeter swan
in flight. As Simppa
has noted below, the program was terminated
before production for several reasons.
For one, there was indeed a crash of a prototype. However, this was not due to a flaw in the design; during a photo flight, one of the accompanying observer aircraft ventured too near the port wingtip of the XB-70 and was caught in the vortex of disturbed air rushing around the outside of the canard (known as wake turbulence). The F-104 was sucked over the top of the larger plane's wing, rolling to the right, and struck one of the Valkyrie's two vertical stabilizers. After being swept over the top of the larger aircraft, the F-104 struck the other vertical stabilizer as well, killing its pilot, Joe Walker. The Valkyrie continued to fly straight on for a few moments before starting to gently roll to the right; when the flight crew attempted to recover the aircraft, it had lost too much aerodynamic performance and stability and went into a nasty spin, progressing to a flat spin, and struck the desert killing the copilot, who was unable to eject in time. The pilot escaped.
There is a series of photographs of the event taken by another photo plane; they are easily found in any large-format book which concerns the XB-70.
This, however, was not the primary reason the airplane was cancelled. At the time, the war in Vietnam was ramping up, and military spending was in a crunch in order to pay for expanded operations in the Southeast Asian theater. The XB-70 would have been enormously expensive to produce, and (much like the B-1 and B-2 were at first) would have been useless in conventional bombing operations. The then-current bomber project, the B-52 series, was readily adaptable to the conventional role and was (with the introduction of the B-52H) becoming the standard heavy bomber for the Air Force's conventional needs. This was most efficient, as its sisters the B-52D and B-52G remained in the force as the nuclear bomber platform, allowing for common maintenance and supply.
Finally, the XB-70 had been designed and built on the notion that 'the bomber will always get through,' a notion born from the performance of the Boeing B-29 Stratofortress, which was able to fly high enough and fast enough to avoid interception by fighters - and with nuclear gravity bombs, the resultant accuracy loss wasn't really relevant. However this notion of airborne invulnerability had recently been dealt a sharp blow by the downing of Francis Gary Powers and his U-2 spyplane by a Soviet SAM. Once it became clear that despite the Mach 3+ speed of the XB-70 it remained vulnerable to surface-fired missiles and modern jet interceptors, the airplane began to look less and less like a capable platform given its somewhat limited bombload and high expense. During this time, the approved nuclear bombing doctrine switched to low altitude penetration and terrain-following, a maneuver the slower but less wing-loaded B-52s were more adept at. The XB-70's enormous swept delta wing meant that it was unsuitable, ungainly and extremely inefficient to fly at low speeds and altitudes.
And so the airplane was never built. It is really a pity; it was a stunning airplane.
There is a postscript, however; even without serving, the XB-70 served. The Soviet Air Force had a fairly good idea of what the XB-70 was capable of, and it scared them quite badly. In point of fact, the missiles that had downed Powers, while able to reach the altitudes of the XB-70, were not very useful against high-speed targets, and the USSR had no interceptors which could operate at the altitude of the U-2 much less the Valkyrie's performance envelope.
As a result, they began a crash program to defeat the Valkyrie. The result was the Mikoyan-Gurevich MiG-25, code-named by NATO the Foxbat. This interceptor was designed for one thing: speed. Capable of Mach 3+ operation, it had limited endurance, limited armament, mediocre maneuverability and poor maintenance characteristics. However, it was the only airplane in general service which could have caught and engaged a Valkyrie at full honk. It set numerous records; the U.S. tried for years to beat its time-to-altitude record, set with a production MiG-25. The mark was bested with the F-15 Streak Eagle, a purpose-built speed version of the F-15 fighter. All nonessential systems were stripped off the aircraft, lightweight substitute parts were emplaced in some cases, and the aircraft was run up to full throttle while bolted to the runway. The explosive bolts were detonated and the Streak Eagle, engines already at full output, barely beat the MiG-25's time to altitude record.
Then the MiG-25 took it back, and keeps it to this day.
The effort required to design and build the MiG-25 was massive, however; and these planes really weren't much use for anything else save reconnaissance. In this sense, the XB-70 contributed quite nicely to the economic front of the Cold War.