Boeing 307 Stratoliner

Development

In 1935, Pan American Airways, United Airlines, American Airlines, Eastern Air Lines, and Transcontinental & Western Air (TWA) had each signed a contract with Douglas to develop the forty-passenger DC-4 (later known as the DC-4E). Each company contributed $100,000 to development costs and agreed to not operate other aircraft with a maximum weight of 43,000 to 75,000 lb (20,000 to 34,000 kg) for revenue service. Due to development problems and poor performance, all of the airlines dropped out of the DC-4 program and cancelled their orders, but a requirement for a large four-engine airliner remained.

Donald Webb Tomlinson at TWA carried out five years of high altitude flight research, with a Northrop Gamma and a Douglas DC-1, which helped determine that TWA would need a four-engine airliner with a pressurized cabin. During this period, he also test-flew the XB-17 and determined that it would provide an ideal basis for an airliner, and so Boeing was approached with the idea. A ceiling of at least 16,000 ft (4,900 m) was required both to avoid summertime "chop" over the Rocky Mountains and to allow the aircraft to fly around the thunderstorms that can sometimes block mountain passes, which meant a pressurized cabin would be the most comfortable for passengers on long flights. In 1935, Boeing then designed a four-engine airliner using components from the Boeing Model 299 B-17 Flying Fortress heavy bomber as the Model 307. It combined the wings, tail, rudder, undercarriage, and engines from the B-17 with a new, much larger pressurized circular–cross-section fuselage with a maximum diameter of 138 in (3.5 m).

The pressurization system required extensive testing, which was carried out over many months, progressively increasing the air pressure after each successful test, and each time, the highly polished fuselage was coated with soapy water while the fuselage was pressurized, for workers to look for the bubbles that would indicate a leak, much like testing a bicycle inner tube.

Before the first aircraft had rolled out, TWA's chief engineer discovered that the extruded metal tubing used for the wing spars was defective, with stress corrosion cracks produced by cold rolling the tubing to increase tensile strength. Affected aircraft included the prototype, the first Pan Am machine, and some early production B-17s, and this resulted in TWA having to have their engineers inspect every tube that was to be incorporated into their aircraft.

The first aircraft completed, registration NX19901, crashed on March 18, 1939, while being demonstrated after having been recently fitted with instruments to measure flight control forces. The Boeing 307 took off at 12:57 pm (local time) from Boeing Field in Seattle with ten occupants, and they climbed to an altitude of 11,000 ft (3,400 m), where stability tests were made and, while carrying out side-slips near Alder, the aircraft stalled and entered a spin. It made two to three turns before the pilot was able to stop the spin using the engines, however, the ensuing high-speed dive and the forces that resulted from attempting to pull up before hitting the ground resulted in the left outer wing tearing off with one engine still attached, followed by the right wing, just outside the outer engine, both of which also tore off parts of the tail as a result of the aileron cables pulling them against the fuselage, which then caused the aircraft to pancake into a forested area at 1:17 pm. All ten aboard were killed, which included TWA's representative, KLM's technical director, a Dutch Air Ministry representative, and Boeing's test pilot, their Chief Aerodynamicist, and their Chief Engineer. Parachutes were available but the force of the spin prevented their use.

The crash delayed the program by over a year, beginning with a three-month investigation by the US Civil Aeronautics Authority, the precursor to the current Federal Aviation Administration, and Boeing to determine the causes. Flight and wind tunnel testing showed that both an extended dorsal fin and an enlarged vertical tail were needed to prevent the rudder stalling in a yaw, and solutions were flight-tested on NC19903, including an intermediate solution consisting of just an extended dorsal fin. The resulting redesign was also incorporated into the redesign of the rear fuselage of the B-17E bomber. The wings were reinforced and Handley Page slots were added to the outer wing leading edges to improve low-speed aileron control while the inboard flaps were also extended. The first several aircraft, including NC19902, NC19903, and NC19904, were all rolled out with the small tail and then modified later.

Test flights resumed on May 19, 1939, after the changes had been incorporated, and on June 20, 1939, the first flight was made with the "supercharged cabin" pressurization system on.

On March 13, 1940, Approved Type Certificate (ATC) number 719 was assigned to the Pan Am Boeing 307s, allowing commercial deliveries to commence.