The Northrop X-4 Bantam was a small twin-jet airplane that had no horizontal tail surfaces, depending instead on combined elevator and aileron control surfaces (called elevons) for control in pitch and roll attitudes.
The hope of some aerodynamicists was that eliminating the horizontal tail would also do away with stability problems at transonic speeds resulting from the interaction of supersonic shock waves from the wings and the horizontal stabilizers.
Many engineers believed in the 1940s that such a design, without horizontal stabilizers, would avoid the interaction of shock waves between the wing and stabilizers. These were believed to be the source of the stability problems at transonic speeds up to Mach 0.9.
Two aircraft had already been built using a semi-wingless design - the rocket-powered Me-163 Komet flown by Germany in World War II, and the British de Havilland DH.108 Swallow built after the war.
The United States Army Air Forces signed a contract with the Northrop Aircraft Company on June 11, 1946, to build two X-4s. Northrop was selected because of its experience with flying wing designs, such as the N-9M, XB-35 and YB-49 aircraft.
The resulting aircraft was very compact, only large enough to hold two J30 jet engines, a pilot, instrumentation, and a 45-minute fuel supply. Nearly all maintenance work on the aircraft could be done without using a ladder or footstool. A person standing on the ground could easily look into the cockpit. The aircraft also had split flaps, which doubled as speed brakes.
The initial NACA X-4 flights, which continued from late 1950 through May of 1951, focused on the aircraft's sensitivity in pitch. NACA pilots Griffith and Scott Crossfield noted that as the X-4's speed approached Mach 0.88, it began a pitch oscillation of increasing severity, which was likened to driving on a washboard road.
Increasing speeds also caused a tucking phenomena, in which the nose pitched down. More seriously, the aircraft also showed a tendency to "hunt" about all three axes.
This combined yaw, pitch and roll, which grew more severe as the speed increased, was a precursor to the inertial coupling which would become a major challenge in the years to come.
The X-4's primary importance involved proving a negative, in that a swept-wing semi-tailless design was not suitable for speeds near Mach 1.
Aircraft designers were thus able to avoid this dead end. It was not until the development of computer fly-by-wire systems that such designs could be practical. Semi-tailless designs appeared on the X-36, Have Blue, F-117, and Bird of Prey, although these aircraft all differed significantly in shape from the X-4.