The Boeing Model 307 Stratoliner was the first commercial transport aircraft to enter service with a pressurized cabin. This feature allowed the aircraft 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). 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, designed to allow pressurization.

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), prompting Boeing to begin production on an initial batch of the airliner.
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. TWA converted their 307s to military service in January 1942, and its Intercontinental Division (ICD) then operated these C-75s under contract to the Army's Air Transport Command (ATC) until July 1944. 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 painted military olive drab.

Two main routes were flown: Washington, D.C., to Cairo across the South Atlantic, and New York to Prestwick, Scotland, across the North Atlantic. They often flew non-stop the 2,125 statute miles (3,415 km) between Gander, Newfoundland and Prestwick, Scotland in the north, and the 2,550 statute miles (4,100 km) between Natal, Brazil and Accra, Ghana in the south. After July 1942 a refueling stop at Ascension Island was an option in the south. In the north, stops at Iceland or Greenland were often necessary, especially flying westbound against the prevailing winds. As C-54s took over the Gander to Prestwick route, the C-75s operated a Marrakech-to-Prestwick service out over the Atlantic.




In April 1945, the five C-75s were returned to TWA, having been restored by Boeing and recertified by the CAA as SA-307B-1 civilian transports with their old registration numbers. TWA then restyled the interior cabin in two sections, ten seats forward and 28 aft.
Operational history
The maiden flight of the first Boeing 307 Stratoliner (not a prototype, as it was planned to be delivered to Pan Am following testing and certification), registration NX 19901 took place from Boeing Field, Seattle on December 31, 1938.




This aircraft crashed on March 18, 1939, while being demonstrated to representatives of KLM. After takeoff the aircraft climbed to an altitude of 11,000 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 11,000 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. The ten people aboard, including the KLM technical director, a representative of the Dutch Air Ministry, a Boeing test pilot, the Boeing Chief Aerodynamicist, the Boeing Chief Engineer, and a TWA representative were killed. Subsequent wind tunnel testing showed that the addition of an extended dorsal fin ahead of and attached to the vertical tail prevented rudder lock. This was incorporated into the 307's rudder redesign, while also being incorporated in Boeing's rear fuselage redesign for their models "E" through "G" B-17 bomber.

The first delivery to a customer was to multi-millionaire Howard Hughes, who bought one 307 for a round-the-world flight, hoping to break his own record of 91 hours 14 minutes set from July 10?14, 1938 in a Lockheed 14. Hughes' Boeing Stratoliner was fitted with extra fuel tanks and was ready to set out on the first leg of the round-the-world attempt when Nazi Germany invaded Poland on September 1, 1939, causing the attempt to be abandoned. This 307 later had the extra fuel tanks removed, was fitted with much more powerful Wright R-2600 engines, and was transformed into a luxurious "flying penthouse" for Hughes, although it was little used, eventually being sold to oil tycoon Glenn McCarthy in 1949.

Deliveries to Pan Am started in March 1940, with TWA receiving its first 307 in April. TWA's Stratoliners flew three-stop flights between Los Angeles and New York while Pan Am's flew from Miami to Latin America. Ten 307s were built, three being delivered to Pan-Am (Clipper Flying Cloud, Clipper Comet, and Clipper Rainbow) and five to TWA (Comanche, Cherokee, Zuni, Navajo, and Apache) with one aircraft going to Hughes.

On the entry of the United States into World War II, Pan Am continued operating its Stratoliners on routes to Central and South America, but under direction of the Army Air Forces, while TWA's 307s were sold to the United States government, being designated Boeing C-75 and operated by the United States Army Air Forces (although normally still flown by TWA crews).

The Army returned its five C-75s to TWA in 1944, who sent them back to Boeing for rebuilding. Boeing replaced the wings and horizontal tail with those from the B-17G, while more powerful engines were fitted and the electrical system was replaced with one based on the B-29 Superfortress. Passenger capacity was increased from 33 to 38. The total rebuilding cost to TWA was $2 million; the five aircraft re-entered passenger service on April 1, 1945. Although TWA was committed to the larger and faster Lockheed Constellation, it kept the Stratoliners until April 1951.
An Aigle Azur Stratoliner at Paya Lebar in 1967

TWA sold its Stratoliner fleet to the French airline Aigle Azur who used them on scheduled flights from France to North and Central Africa, and later to French Indo-China. These 307s were later transferred to Aigle Azur's Vietnamese subsidiary and were used by a number of airlines in South East Asia, with at least one aircraft remaining in commercial use until 1974.

Pan Am flew its unmodified 33-passenger Stratoliners between Miami and Havana until 1947, then sold them to small operators. One aircraft was purchased by the Haitian Air Force, being fitted as a Presidential transport for Fran?ois "Papa Doc" Duvalier. This aircraft later returned to the U.S. and was purchased by the Smithsonian Museum.
Below Cockpit & interior



Below Pan Am












Below TWA










Below Aigle Azur etc






Specifications (Boeing 307)
General characteristics
Crew: five: two pilots, flight engineer, two cabin crew
Capacity: 38 passengers in daytime, 25 by night
Length: 74.3 ft (22.6 m)
Wingspan: 107 ft (33 m)
Height: 20.79 ft (6.34 m)
Wing area: 1,486[4] sq ft (138.1 m2)
Empty weight: 30,000 lb (13,608 kg)
Gross weight: 45,000 lb (20,412 kg)
Powerplant: 4 ? Wright GR-1820-G102A radial engines, 1,100 hp (820 kW) each
Performance
Maximum speed: 241 mph (388 km/h; 209 kn)
Cruise speed: 215 mph (346 km/h; 187 kn)
Range: 1,750 mi (1,521 nmi; 2,816 km)
Service ceiling: 23,300 ft (7,100 m)
Wing loading: 28 lb/sq ft (140 kg/m2)
Power/mass: 0.098 hp/lb