The Boeing Model 307 Stratoliner (or Strato-Clipper in Pan American service, or C-75 in USAAF service) is an American stressed-skin four-engine low-wing tailwheel monoplane airliner derived from the B-17 Flying Fortress bomber, which entered commercial service in July 1940. It was the first airliner in revenue service with a pressurized cabin, which, along with supercharged engines, allowed it to cruise above the weather. As such it represented a major advance over contemporaries, with a cruising speed of at compared to the Douglas DC-3's , at then in service.Davies, 2000, p. 52 When it entered commercial service it had a crew of five to six, including two pilots, a flight engineer, two flight attendants, and an optional navigator and had a capacity for 33 passengers, which later modifications increased, first to 38, and eventually to 60.
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).Dijkstra, 2016, p. 75 Each company contributed $100,000 to development costs and agreed to not operate other aircraft with a maximum weight of 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.Betts, 1990, p. 55 A ceiling of at least 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 .
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.Betts, 1990, p. 57 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 , 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.Dijkstra, 2016, p. 80 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.Air Safety Board Report involving NX19901, of the Boeing Aircraft Company, near Alder, Washington, March 18, 1939 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.Abzug & Larrabee, 2005, p. unk. 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.Dijkstra, 2016, p. 81
On March 13, 1940, Approved Type Certificate (ATC) number 719 was assigned to the Pan Am Boeing 307s, allowing commercial deliveries to commence. A second ATC was issued for the TWA aircraft, number 726, due to the numerous differences between the Pan Am and the TWA aircraft.
TWA was concerned about excessive undercarriage stiffness from their experience with the test flights with the XB-17, which were to be modified for the airliner. TWA flight-tested the modified undercarriage and in hard landings easily exceeded the contract's minimum required descent rate with a successful landing descent.Betts, 1990, p. 67
TWA was also concerned about the engine carburetor intake heaters being deliberately restricted by Boeing to prevent cooking the engines, which could potentially leave crews unable to clear ice.Betts, 1990, p. 69 Their point was made when icing problems during a test flight on May 17, 1940, with NC19905, while carrying dignitaries in overcast conditions over mountains resulted in three of the four engines failing, while the fourth was losing power, despite every measure being taken to clear the ice. This resulted in the aircraft making a belly landing in a field with a partially lowered undercarriage, just south of Lamar, Colorado.Betts, 1994, p. 51 TWA then modified the carburetor heating themselves and the aircraft was repaired and returned to service.
Boeing made claims both in their period advertising and in their current web site that it was the first high-altitude commercial transport and the first with a flight engineer.Boeing, Historical Snapshot, 1995–2022 However, its first flight, on December 31, 1938, was later than that of the Renard R-35, which also had a pressurized cabin for passengers and flew on April 1, 1938, but that craft crashed and its development was abandoned.De Wulf, 1978, pp. 147–149 As for employing a flight engineer, it was preceded in the US on a commercial aircraft by Maddux Air Lines Ford Trimotors, whose "Mate" had the same responsibilities as a flight engineer. Additionally, all German World War One Riesenflugzeug multi-engine bombers had flight engineers as they were integral to the specification.Haddow, 1962, p. 2
Design
As built, the Stratoliner used the all-metal stressed-skin cantilever wings from the B-17C mounted low on the fuselage to a constant-chord center section faired to the fuselage,Juptner, 1980, p. 102Juptner, 1980, p. 104 with four Wright GR-1820 Cyclone air-cooled radial engines. TWA examples used GR-1820-G105A engines fitted with two-stage superchargers for high-altitude performance, while the Pan Am examples used the GR-1820-G102 with a single-stage supercharger.Juptner, 1980, p. 103 To reduce noise, engine exhaust collector rings and mufflers were installed. TWA aircraft were fitted with cowl flaps, to adjust engine cooling air, while Pan Am aircraft had fixed cowling rings without cowling gills. When operating in cooler conditions, the Pan Am aircraft could be fitted with blanking disks that partially covered the fronts of the engines. Both versions had sufficient power to maintain altitude on only two engines, one of the KLM requirements. Both used three-bladed Hamilton Standard Hydromatic constant-speed propellers, and new high-octane fuels were developed to help the engines operate under the increased supercharger pressure. Both versions had trailing-edge flaps controlled with electric motors, although the SA-307B, for TWA, and the SB-307B, for Hughes, featured slotted flaps with prominent external hinges, while the Pan Am examples had simpler split flaps with flush hinges, similar to those used on the B-17s.Minshall, 1940, p. 116Betts, 1990, p. 63 All of the fuel was carried in the wings, with, in each wing, a tank between the inboard nacelle and the fuselage, a main tank, and a tank between the inner and outer nacelles, making a total capacity of in six tanks. With the fuselage being wider than that of the B-17, the span had increased from to compared to early B-17s.Betts, 1990, p. 59 After being modified, the SA-307B-1s used the wings and elevators from the B-17G, with split flaps, and Cyclones. On most, but not all, examples, the leading edges of the wings, horizontal stabilizer, and fin were fitted with rubber expanding-type de-icing boots, which would inflate and deflate repeatedly to break ice from the flying surfaces. All movable surfaces, including the rudder, ailerons, and elevators had fabric over metal structures and were aerodynamically balanced and fitted with adjustable trim tabs to lighten flight loads.McLarren, 1938, p. 40 The rudder and elevators also had hydraulic boost to lighten control forces. The partially retractable main undercarriage had hydraulic brakes, used Goodyear 55×19×23 tires, and was raised and lowered with electric motors. Manual backups were provided for electrically driven systems, but the power had to be turned off before being used.Betts, 1990, p. 73 A parking brake was provided, along with an emergency air brake system run off a bottle of compressed air, while the tailwheel was fully retractable.