The X-4, a single-place, low swept-wing, semi-tailless aircraft was designed and built by Northrop Aircraft, Inc., for NACA. It had no horizontal tail surfaces and its mission was to obtain in-flight data on the stability and control of semi-tailless aircraft at high subsonic speeds. The two X-4 aircraft were 23 ft long, 14 ft high, and had wingspans of 26 ft. They weighed almost 8,000 lb at takeoff.
The X-4 was powered by two Westinghouse XJ-30 turbojet engines with 1,600 lb of thrust each. These engines boosted the X-4 up to speeds of 620 mph and up to altitudes of 40,000 ft. The aircraft's maiden flight was on Dec. 16, 1948, with Charles Tucker, a Northrop test pilot, at the controls.
The X-4 helped demonstrate that tail surfaces are important for proper control effectiveness in the transonic speed range, and was also used to investigate the characteristic problems of tailless airplanes at low speeds, such as marginal longitudinal stability and control
