The NACA High-Speed Flight Research Station X-4 research aircraft is seen in this 1950s in-flight close-up photograph. The two large ''X''s seen connected by a line painted on the aircraft were used as an aid in optical tracking. The X-4 Bantam, a single-place, low swept-wing, semi-tailless aircraft, was designed and built by Northrop Aircraft, Inc. 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.