The XB-70 was built using stainless steel honeycomb sandwich construction, which was very complex and expensive. It was also extremely vulnerable to being tracked by radar. The shape had a lot of right angles that turned the airplane into a huge radar reflector.

The Blackbird was designed to be fairly stealthy, particularly in the wavelengths used by Soviet long-range tracking radar. When he designed the airplane, Kelly Johnson rejected stainless steel honeycomb in favor of less complex titanium structures. He conceived it as a "universal airplane" that could be produced in several different versions for different roles (reconnaissance, interceptor, bomber, etc.). It would have some common design elements and others that could be changed in accordance with the desired mission.


For the record, we do know the actual Blackbird performance specifications:

The maximum design cruise speed was Mach 3.2. Speeds in the Mach 3.3 range were recorded during test flights. Maximum speed was limited by structural temperature restrictions (compressor inlet temperature had to remain below 427 degrees Centigrade).

Fastest known flights:

YF-12A (60-6936) – Mach 3.14 (2,070 mph), USAF, official, 1 May 1965
SR-71B (61-7956) – Mach 3.27 (2,158 mph), NASA, unofficial, 14 December 1995
A-12 (60-6928) – Mach 3.29 (2,171 mph), CIA, unofficial, 8 May 1965
SR-71A (61-7958) – Mach 3.32 (2,193 mph), USAF, official, 28 July 1976


The Blackbirds were designed to fly as high as 90,000 feet, but typically operated between 70,000 and 85,000 feet. The maximum altitude recommended in the SR-71 flight manual was 85,000 feet. SR-71 The A-12, with its lighter airframe was capable of cruising at 90,000 feet.

Highest known flights:

YF-12A (60-6936) – 80,257 feet, USAF, official, 1 May 1965
SR-71B (61-7956) – 84,700 feet, NASA, unofficial, 18 October 1994
SR-71A (61-7962) – 85,068 feet, USAF, official, 28 July 1976
SR-71A (61-7953) – 86,700 feet, USAF, during performance testing, 1968
SR-71A (61-7953) – 89,650 feet, USAF, unofficial, 1968
A-12 (60-6932) – 90,000 feet, CIA, unofficial, 14 August 1965

If you read "Skunk Works" Ben Rich states that the XB-70 was cancelled because the Kennedy administration thought the Blackbird could be made into a bomber - Lockheed had plans for all kinds of SR-71 versions - including an interceptor and a bomber version ready to go.

Six engined behemoth vs two engined semi-stealthy plane with better performance. Its a no brainer.

Not only that, but the "missile gap" was proved an illusion and drive towards intervention in Vietnam meant that the money needed to go elsewhere.