Boeing 307 Stratoliner

Boeing 307 Stratoliner
C-75
Boeing 307 Udvar Hazy.jpg
A restored (ex-Pan Am) Boeing 307 on display at the Steven F. Udvar-Hazy Center
RoleAirliner
ManufacturerBoeing
First flightDecember 31, 1938
IntroductionJuly 4, 1940 with Pan American Airways[citation needed]
Retired1975
StatusRetired
Primary usersTWA
Pan American Airways
United States Army Air Forces
Number built10
Unit cost
$315,000 in 1937[1] (equivalent to $5.5 million today)
Developed fromBoeing B-17 Flying Fortress

The Boeing Model 307 Stratoliner was a commercial transport aircraft that entered service in 1938. It was the first to offer a pressurized cabin, allowing it to cruise at an altitude of 20,000 ft (6,000 m), well above many weather disturbances. The pressure differential was 2.5 psi (17 kPa), so at 14,700 ft (4,480 m) the cabin air pressure was equivalent to an altitude of 8,000 ft (2,440 m). The Model 307 had capacity for a crew of six and 33 passengers. The cabin was nearly 12 ft (3.6 m) across. It was the first land-based aircraft to include a flight engineer as a crew member (several flying boats had included a flight engineer position earlier).[1] In addition to its civilian service it was also flown as the Boeing C-75 Stratoliner by the United States Army Air Forces, who used it as a long-range cargolift aircraft.

Development and design

In 1935, Boeing designed a four-engine airliner based on its B-17 heavy bomber (Boeing Model 299), then in development, calling it the Model 307. It combined the wings, tail, rudder, landing gear, and engines from their production B-17C with a new, circular cross-section fuselage of 138 in (351 cm) diameter,[2] designed to allow pressurization.[3]

The first order, for two 307s (named Stratoliners), was placed in 1937 by Pan American Airways. Pan Am soon increased this to six, and a second order for five from Transcontinental & Western Air (TWA) prompted Boeing to begin production on an initial batch of the airliner.[3][4][5]

C-75 conversion

At the time the United States entered World War II in December 1941, flying across oceans was a rare luxury. The war required government and military officials to do so, and most four-engined long-range commercial aircraft, including Pan American Airways' 14 flying boats and TWA's five Boeing 307s, were pressed into service. Additional fuel tanks were added to give them the extra range required; once converted they were designated C-75 for military use. Before World War II ended their production, ten commercial 307s had been built. TWA flew domestic routes between New York and Los Angeles for 18 months until the Army purchased their Stratoliners for wartime use as long-range, transatlantic transports for various VIPs or critical cargo on 26 January 1942.[6] TWA converted their 307s to military service in January 1942,[7][page needed] and its Intercontinental Division (ICD) then operated these C-75s under contract to the Army's Air Transport Command (ATC) until July 1944.[2] These were the only U. S. built commercial aircraft able to cross the Atlantic with a payload until the arrival of the Douglas C-54 Skymaster in November 1942.

Conversion to the C-75 included removal of the pressurization equipment to save weight, removal of the forward four (or five) of nine reclining seats along the port side, and alteration of the two forward Pullman-like compartments (of four) starboard of the left-of-centerline aisle. Space was thus provided for crew requirements on extremely long flights and for the addition of five 212.5 U.S. gal (804 L; 177 imp gal) fuel tanks. The landing gear was strengthened, the maximum takeoff weight was increased from 45,000 to 56,000 lb (20,400 to 25,400 kg), and the exterior was painted military olive drab.[2]

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