Superfortresses and Rocket PlanesThe United States developed the first rocket propelled, transonic research aircraft after World War II. Bell Aircraft developed the X 1 under contract to the Army Air Force. Douglas Aircraft developed the D 558 II Skyrocket under contract to the Navy. Due to the low power of the jet engines available at the time, both research aircraft were to be powered by rocket engines built by Reaction Motors Incorporated. The Bell X 1 used the XLR 11 and the