Part II : 1950-1957
 The initial contract with Lockheed called for two prototype reconnaissance aircraft, with the first to fly in 18 months. Hard on its heels, also in 1956, Lockheed received a contract for six of the aircraft. The design Lockheed selected was designated CL-400 and was capable of a speed of Mach 2.5 at an altitude of 30 300 meters.11 The CL-400 was described openly for the first time in 1973 by Ben Rich at a symposium on hydrogen-fueled aircraft at the NASA Langley Research Center. Figure 34, taken from his paper, shows the characteristics of the CL-400. It had a fuselage diameter of three meters and a length of 49 meters to accommodate the 9740 kilograms of liquid hydrogen. The retractable ventral (bottom) fin improved directional stability at supersonic speeds.
The engines, designated 304-2, were to be supplied by Pratt & Whitney and will be described later. Each weighed 2850 kilograms, provided 42 kilonewtons at sea-level, and 27 at Mach 2.5 and 29000 meters altitude.
The mission profile is shown by figure 35. The range was 4070 kilometers and could be extended only by a considerable increase in airplane size. Airplane sizes with lengths as long as a football field, as well as other variables, were studied at the Skunk Works. The relatively short radius of 2000 kilometers was later to become a matter of great concern.