On 5 June 1948, Maj. Daniel Forbes piloted 368 on a routine test flight. Capt. Glen W. Edwards was the co-pilot. The flight engineer was Lt. Ed Swindell, and there were two civilian observers aboard, Claire C. Leser and Charles H. LaFountain. The aircraft departed Muroc Air Force Base in the early morning. Routine position reports over the Antelope Valley test range were received for about an hour. The final radio transmission came at the beginning of a test run to measure the longitudinal control forces at increasing angles of attack. The aircraft was flying at an altitude between twenty and twenty-five thousand feet. About 20 minutes after the last report, witnesses reported seeing the YB-49 crash in the desert north of the base.

The YB-49 had crashed on top of some hills 10 miles east of Mojave. It was determined that the jet had impacted in a flat attitude with little forward velocity. At thirty-five minutes into the flight it still had considerable fuel, and when it struck the ground, it exploded and burned. Parts of the airplane, including outer wing panels and flap and elevator components, were found in a narrow area extending two or three miles from the wreckage. All five crewmembers were killed. They had apparently made no attempt to bail out.

During the accident investigation, it was impossible to arrive at an incontestable conclusion regarding the probable cause. A major structural failure had obviously occurred in flight. Many specific causes were considered: exceeding flight restrictions, tumbling resulting from a stall, instability due to extreme aft center of gravity due to improper fuel usage, unintentional nose-up trim flap motion, or hydraulic control system malfunction.

In the final analysis, it appeared that the crash was caused by simultaneous failure of both outer wing panels from a positive loading condition. The failure could have been due to loads caused by a high-G pullout, overstressing the airframe. One of the most plausible explanations was that the overstress occurred during recovery from a stall or post-stall maneuver.