Trans World Airways and Pan American Airways contracted with Boeing for a total of nine of the four-engine aircraft. (A 10th was built for industrialist Howard Hughes.) The Stratoliner was designed with the wings, tail assembly, and engines of the B-17 Flying Fortress, making it the largest land based airplane in the world at that time. The most attractive feature was a pressurized cabin, allowing the plane to fly at 26,000 feet at the comfort of sea level, and over weather systems. Boeing engineers used a converted 55-gallon oil drum to develop an innovative pressurization valve because the company could not afford a conventional pressure chamber.
The Dutch airline KLM expressed an interest in the Model 307. KLM test pilot Albert von Baumhauer and another engineer traveled to Seattle for an evaluation. Von Baumhauer wanted to test the Stratoliner at low speed with engines on one side shut down. Despite the low likelihood of such a combination of conditions, Chief Aerodynamcist Ralph Cram agreed to the test.
According to the findings of the accident investigation, the aircraft was at 10,000 feet when the maneuver was attempted. The airplane stalled and went into a spin. This failure was traced to ailerons and a tail fin too small for this design (plans to modify the original bomber flight surfaces had already been finalized, but had not been incorporated into the prototype).
Boeing test pilot Julius Barr and von Baumhauer attempted to recover from the spin, but their struggle against the control column resulted in the wings and tail section separating from the fuselage. Also killed in the crash was Boeing Chief Engineer Earl Ferguson, a representative from TWA, and five other Boeing employees.
One immediate result of this tragedy was the formation of a department of aerodynamics and flight research. According to Chief Test Pilot Eddie Allen (d. 1943), the department's first director, "The day when you build an airplane and call a pilot like me to test it is over."
Fast and Luxurious
Nine Stratoliners entered service in the U.S. and on Pan Am's Latin American routes beginning in July 1940. The Model 307 cut the transcontinental air trip to 14 hours and nine minutes, two hours faster than the twin engine DC-3. Because the pressurized cabin allowed 33 day passengers or 26 overnight passengers, and the comfort of sea level while flying over weather systems, the Stratoliner was very popular with the traveling public. Flights were often fully booked. However, troubled by the Mount Rainier crash and an emergency landing by TWA in Colorado in 1940, U.S. airlines purchased the Douglas DC-4 instead, even though it was underpowered and unpressurized.
After just 18 months of domination of U.S. air travel, the Stratoliners were taken into the U.S. Army Air Corps Air Transport Command for service in World War II. They were stripped of their pressurization equipment and finely appointed interiors.
The Boeing Company shifted design and production resources to bombers.