The Boeing S.307 Stratoliner prototype was destroyed when it broke up during a test flight out of Seattle-Boeing Field, USA. All 10 on board were killed. Dutch airline KLM had expressed an interest in the newly developed Boeing S.307. The KLM technical director and a representative of the Dutch Air Ministry, Albert von Baumhauer, had traveled to Seattle for an evaluation of the aircraft. Von Baumhauer had particular interest in the stability and control of the aircraft. The aircraft took off from Boeing Field, Seattle at 12:57 on test flight no. 19. The captain occupied the left hand cockpit seat, Mr. von Baumhauer occupied the right hand seat. Mr. von Baumhauer held a private pilot’s license and his total flying time as pilot amounted to 116 hours. He had no experience as pilot or co-pilot of four-engine aircraft, but had been observer in trial flights of four engine Fokker F.22 and F.36 aircraft. After takeoff the aircraft climbed to an altitude of 11000 feet. At this altitude longitudinal stability tests were made. The next tests, as outlined by the flight plan, were side-slip tests. The aircraft went into an inadvertent spin subsequent to a stall at an altitude of approximately 11000 feet. It made two to three turns in the spin, during which the engines were used to aid recovery. In recovering from the dive subsequent to the spin, the wings and horizontal tail surfaces failed upward apparently due to air loads in excess of those for which the aircraft was designed. Investigators noted that the captain on the flight was inexperienced in testing and demonstrating aircraft of this type, particularly with respect to the maneuvers called for. Also, Mr. Von Baumhauer, acting as copilot was inexperienced. Other potential factors were the sensitivity of the elevator and rudder control system, possible elevator flutter and insufficient vertical tail surfaces to give adequate control of the aircraft under all conditions of flight. The tail surface was changed and enlarged in production models. PROBABLE CAUSE: “Structural failure of the wings and horizontal tail surfaces due to the imposition of loads thereon in excess of those for which they were designed, the failure occurring in an abrupt pull-out from a dive following recovery from an inadvertent spin.”