A pair of prototypes were completed, known as the "X-1" and the "X-2". The X-1 was to be outfitted with an arrangement of six RB145 engines: two being mounted vertically in the fuselage for lift and four within the swivelling nacelles, each of which being able to generate 2,750 lbf of thrust. The X-2 was to have the swivelling engines equipped with an afterburner, which would enable them to produce a wet thrust of 3,840 lbf each. In turn, this was projected to enable the aircraft to attain its design speed of Mach 1.8.