What happened
A flight involving a crew of six, consisting of pilots and engineers from Boeing as well as United States Air Force officers, was conducting a local test flight departing from Seattle-Boeing Field. The purpose of the mission was to evaluate recent technical modifications performed on the four-engine aircraft.
During the initial climb phase shortly after departure, the aircraft experienced significant difficulty maintaining altitude. While struggling to gain height, the right wing of the plane struck the roof of a brewery situated approximately one mile north of the airfield. This impact caused the aircraft to lose control, leading to an aerodynamic stall. The plane subsequently crashed into the Lester Apartments building, where a massive explosion occurred. The impact and a subsequent fire, fueled by roughly 4,000 gallons of onboard fuel, completely destroyed the airframe.
Findings
The accident resulted in the deaths of all six crew members and five residents of the apartment building. Additionally, 11 people on the ground sustained injuries. While the investigation identified the wing strike as the immediate event leading to the crash, the specific underlying reason for the loss of altitude remains undetermined.