The X-1A, B, and D were essentially identical rocketplanes intended to reach speeds above Mach 2. The X-1A and X-1D were destroyed in in-flight explosions; the X-1B survived, training future moon lander Neil Armstrong on its last flights in 1958.

Main Engine: XLR-11. Main Engine Propellants: Lox/Alcohol.

• 1951 July 24 - X-1D Flight 1 - Crew: Ziegler.
  Bell flight 1. Glide flight for familiarization. Nose landing gear broken on landing. Following repairs, plane turned over to the Air Force.

• 1951 August 22 - X-1D Flight 2 - Crew: Everest.
  AF flight 1. Launch aborted, but X-1D suffered low-order explosion during pressurization for fuel jettison. Plane jettisoned from B-50. X-1D exploded on impact with desert. Everest managed to get into B-50 bomb bay before drop.