The X-26A was an Schweizer SGS 2-32 sailplane that was used by the Navy to expose novice pilots to the phenomenon of yaw/roll coupling. Conventional jet trainers reacted much too quickly and dangerously for effective instruction - an aircraft that had unusually slow roll rates and excellent recovery characteristics was needed instead. Four of the gliders were originally delivered, but accidents soon claimed three of them. In each case the aircraft was replaced with a new one, and the training program continued, making the X-26 the longest-lived X-vehicle.
The Lockheed X-26B was created in response to a requirement for a stealth-type observation aircraft in Vietnam. Two of the Navy X-26A aircraft were temporarily modified with small engines and slow-speed propellers, and were eventually equipped with a variety of intelligence-gathering sensors. Testing in Vietnam was evidently successful as 14 further aircraft were acquired under various designations, including 11 Lockheed YO-3As.