The Central Intelligence Agency has published a history of Oxcart, the super-secret programme that resulted in Lockheed's Mach 3-plus A-12, YF-12A and SR-71 Blackbird.
The monograph, by CIA chief historian David Robarge, provides insight to the competition between Lockheed's A-12 and Convair's Kingfish - a remarkable-looking aircraft powered by the same Pratt & Whitney J58 engines, which promised lower radar cross-section but was judged to be higher risk.
Thanks to the website secretprojects.co.uk you can see some amazing artwork of the Kingfish by the great Jozef Gatial.
Under the codename Gusto, the CIA in 1956 began looking for a high-speed, high-altitude replacement for the Lockheed U-2. The Skunk Works proposed a number of Archangel, or A-series designs:
Convair proposed the Fish, a manned, Mach 4-plus ramjet-powered parasite aircraft to be launched from under the planned B-58B Super Hustler:
Lockheed continued to evolve its design, eventually arriving at the A-11. This was rejected because its RCS was too large. Convair's Fish was smaller, but cancellation of the B-58B in June 1959 put it out of the running.
Convair and Lockheed submitted new proposals in August 1959: Convair proposed the Kingfish, a ground-launched aircraft powered by two J58s; and the Skunk Works proposed the A-12:
The A-12 had better the performance, but the Kingfish had the smaller RCS. In the end, says Robarge, the Skunk Works' success with the U-2 was measured against Convair's problems with the B-58 and the rest, as they say, is history (which you can also read here).