The United States Douglas Skystreak (D-558-1 or D-558-I) was a single-engine jet research aircraft of the 1940s. It was designed in 1945 by the Douglas Aircraft Company for the U.S. Navy Bureau of Aeronautics, in conjunction with the National Advisory Committee for Aeronautics (NACA)
The D558 program was conceived as a joint NACA/U.S. Navy research program for transonic and supersonic flight. As originally envisioned, there would be three phases to the D558 program: a jet-powered airplane, a mixed rocket/jet-powered configuration, and a design and mockup of a combat aircraft.
All the Skystreaks were initially painted scarlet, which led to the nickname "crimson test tube." NACA later had the color of the Skystreaks changed to white to improve optical tracking and photography.
The Skystreak reached Mach 0.99 in level flight, but only flew supersonic in a dive. In the public mind, much of the research performed by the D-558-1 Skystreaks was quickly overshadowed by Chuck Yeager and the supersonic X-1 rocket plane. However, the Skystreak 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.