AF flight 20. Engine fire after launch forced jettisoning of propellants, completed as a glide flight. However Fitz-Gerald reached 12.1 km and reached Mach 1.10 before the engine was shut off, in the process becoming the second person to break the sound barrier.

During a Bell captive-carry flight test over Lake Ontario, X-2 number 46-675 suddenly exploded, killing Bell test pilot Jean Ziegler and observer Frank Wolko. The EB-50A mothership managed to land, although damaged. Only after several other mysterious X-plane losses was the cause found to be a rocket engine gasket made of Ulmer leather, which decomposed and became explosively unstable after sustained exposure to liquid oxygen.

NACA met with USAF and Navy BuAer representatives to propose the X-15 as an extension of the cooperative rocket research aircraft program. The NACA proposal was accepted as a joint effort and a memorandum of understanding was signed on December 23 naming NACA as technical director of the project, with advice from a joint Research Airplane Committee.