The D-558 program was conceived as a joint NACA/U.S. Navy research program for transonic and supersonic flight. The D-558 Phase One, or D-558-1, was known as the Skystreak while the D-558 Phase Two, or D-558-2, was known as the Skyrocket.
A contract for design and construction of six D558-1 aircraft for the first phase was issued on June 22, 1945. The original plan had been for six aircraft with a mixture of nose and side inlets and varying wing airfoil sections. That plan was quickly reduced to three aircraft of a single configuration with a nose inlet. Plans for the second phase with mixed rocket/jet propulsion were also dropped. Instead, a new aircraft, the D-558-2, was designed with mixed rocket and jet propulsion for supersonic flight.
The Skystreak, or D-558-1, performed an important role in aeronautical research by flying for extended periods of time at transonic speeds, which freed the X-1 to fly for limited periods at supersonic speeds.
The Douglas D-558-2 Skyrocket was used for transonic study and the vehicle Scott Crossfield used to break Mach 2. This craft had three models. Jet powered, Jet/Rocket powered, and Rocket powered.
Toward Mach 2: The Douglas D-558 Program Edited by J. D. Hunley NASA SP-4222
On the Frontier: Experimental Flight at NASA Dryden Richard Hallion and Michael Gorn Smithsonian Books
Skystreak, Skyrocket, & Stiletto Scott Libis Specialty Press
Douglas D-558-2 Skyrocket Scott Libis Ginter Books