|Boeing 307 / C-75|
|A restored (ex-PanAm) Boeing 307 on display at the Steven F. Udvar-Hazy Center|
|First flight||31 December 1938|
|Introduction||8 July 1940|
United States Army Air Forces
|Unit cost||$315,000 (in 1937 when ordered)|
|Developed from||Boeing B-17 Flying Fortress|
The Boeing Model 307 Stratoliner was the first commercial transport aircraft 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 altitude was 8,000 ft (2,440 m). The Model 307 had capacity for a crew of five 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).
Development and design 
In 1935 Boeing designed a four-engined airliner based on its B-17 heavy bomber (Boeing Model 299), then in production, 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 U.S. 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 Am's 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, 10 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 transport for various VIPs or critical cargo. TWA converted their 307s to military service in January 1942,[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. 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 take-off weight was increased from 45,000 to 56,000 lb (20,400 to 25,400 kg)), and the exterior painted military olive drab.
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. It crashed, however, on March 18, 1939, while being demonstrated to representatives of KLM. At the time, the 307's performance with two engines inoperative on one wing was being demonstrated. When the engines were shut down, the pilot moved the rudder to maximum deflection to counter the resulting yaw. The Stratoliner then experienced rudder lock, where the control loads prevented the rudder from being re-centered. As a result, the 307 went into a spin and crashed. The 10 people aboard, including KLM test pilot Albert von Baumhauer, Boeing test pilot Julius Barr, Boeing Chief Aerodynamcist Ralph Cram, Boeing Chief Engineer Earl Ferguson, 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 from happening. 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 long-range bomber.
The first delivery to a customer was to multi-millionaire Howard Hughes, who purchased one 307 for a round-the-world flight, hoping to break his own record of 91 hours 14 minutes set between 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 (named Clipper Flying Cloud, Clipper Comet, and Clipper Rainbow) and five to TWA (named Comanche, Cherokee, Zuni, Navajo, and Apache) with the one aircraft going to Howard Hughes. As previously noted, the first 307 crashed during a flight test demonstration to Dutch airline KLM .
On the entry of the United States into World War II, following the Attack on Pearl Harbor, Pan-Am continued operating its Stratoliners on routes to Central and South America, but under direction of the Army Air Force, while TWA's 307s were sold to the U. S. government, being designated Boeing C-75 and operated by the United States Army Air Forces (although normally still flown by TWA crews).
The U. S. Army returned its five C-75s to TWA in 1944, who then sent them back to Boeing for extensive overhaul and rebuilding. Boeing replaced the wings and horizontal tail with those from the improved B-17G bomber model, while more powerful engines were fitted and the electrical system was replaced with one based on the Boeing B-29 Superfortress. Passenger capacity was increased from 33 to 38. The rebuild cost to TWA was $2 million for the five aircraft, which then re-entered passenger service on April 1, 1945. Although TWA was now committed to the larger and faster Lockheed Constellation, the Stratoliners remained in use for regional passenger service until being withdrawn and sold in April, 1951.
TWA then sold its Stratoliner fleet to the French airline Aigle Azur, who used them for 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 disposed of its unmodified Stratoliners earlier than TWA, with them being sold to a number of small operators. One aircraft was purchased by the Haitian Air Force, being fitted out as a Presidential transport for Papa Doc Duvalier. This aircraft was later returned to the United States and was purchased by the Smithsonian Museum.
Two main routes were flown: Washington, D.C., to Cairo across the South Atlantic, and New York to 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. These were very long flights for the time. 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. The restoration included new B-17G Flying Fortress wings and engines, horizontal stabilizer moved three feet (0.9 m) aft, and improved B-29-type electronics. TWA then restyled the interior cabin in two sections, 10 seats forward and 28 aft.
- original concept designation of 307.
- equipped with Wright Cyclone GR-1820-G102 engines with single speed supercharger; 5 crew.
- equipped with Wright Cyclone GR-1820-G105A engines with two speed supercharger for improved high altitude performance; 7 crew.
- Five Trans World 307Bs were pressed into service with the USAAF as military transports; the cabin pressurization was removed to save weight.
- Following military service, the C-75s were overhauled and updated with B-17G wings and tailplane, four Wright Cyclone GR-1820-G606 engines, and B-29-type electronics.
Civilian operators 
- Aigle Azur ex-TWA aircraft bought in 1951 with replaced B-17G engines and wings.
- Royal Air Laos received ex-Aigle Azur aircraft.
Military operators 
- United States Army Air Forces, the five TWA 307Bs were pressed into service and designated as C-75s. The three Pan Am 307s were also operated for the USAAF during World War II but retained civil registrations and were not re-designated.
Accidents and incidents 
- On March 18, 1939, the first Boeing 307, registration NX19901, crashed at Boeing Field during a demonstration flight for KLM, killing all 10 on board. The aircraft experienced rudder lock and went into a spin. The pilots attempted to recover, but this caused the aircraft to break up.
- On May 10, 1958, a Quaker City Airways Boeing 307, registration N75385, force-landed near Madras, Oregon, after an explosion and fire in the cabin, possibly due to a fuel leak; the aircraft was consumed by the fire. Both pilots survived; the aircraft was on a test flight.
- On May 22, 1961, an Aigle Azur Extrême Orient Boeing 307, registration F-BHHR, was blown off the runway and crashed at Tan Son Nhat International Airport while landing after the number four engine failed in flight; all 28 passengers and crew survived, but the aircraft was written off. The aircraft was operating a non-scheduled Saigon-Vientiane service.
- On December 29, 1962, an Airnautic Boeing 307B-1, registration F-BELZ, crashed into Monte Renoso on Corsica due to crew error while on a Bastia-Nice-Ajaccio-Nice-Bastia flight, killing all 25 passengers and crew on board.
- On October 18, 1965, an ICC Boeing 307B-1, registration F-BELV, crashed near Hanoi en route from Vientiane, killing all 13 on board; the aircraft was possibly shot down.
- On June 27, 1974, a Cambodia Air Commercial Boeing 307B-1, registration XW-TFR, force-landed in a rice field minutes after takeoff from Battambang Airport en route to Phnom Penh due to improper maintenance causing the failure of three engines; the right wing struck a tree and separated just before landing. Nineteen of 39 on board died.
- On March 13, 1975, a Royal Air Lao Boeing 307B-1, registration XW-TFP, force-landed in the Mekong River in Laos while on a Hong Kong-Vientiane cargo service; both pilots (who survived) were held prisoner by the Pathet Lao for several months before being released in May 1975. The aircraft was located in 1986.
The only surviving Boeing 307 Stratoliner (NC19903) is preserved in flying condition at the Smithsonian Museum's Steven F. Udvar-Hazy Center. On March 28, 2002, this particular aircraft was subject to a dramatic crash in which it ditched into Elliott Bay in Seattle, Washington, on what was to be its last flight before heading to the Smithsonian. Despite the incident, it was again restored, flown to the Smithsonian, and is now on display.
Specifications (Boeing 307) 
Data from Jane's AWA 1942 (apart from wing area and loading)
- Crew: 5, including two pilots and flight engineer
- Capacity: 38 passengers in daytime, 25 by night
- Length: 74 ft 4 in (22.6 m)
- Wingspan: 107 ft 0 in (32.63 m)
- Height: 20 ft 9.5 in (6.33 m)
- Wing area: 1,486 ft² (138.0 m²)
- Empty weight: 30,000 lb (13,608 kg)
- Loaded weight: 45,000 lb (20,420 kg)
- Powerplant: 4 × Wright GR-1820-G102 radials, 1,100 hp (820 kW) each
- Maximum speed: 241 mph at 6,000 ft (387 km/h at 1,830 m)
- Cruise speed: 215 mph at 10,000 ft (344 km/h at 3,050 m)
- Range: 1,750 mi (2,820 km)
- Service ceiling: 23,300 ft (7,110 m)
- Wing loading: 28 lb/ft² (138 kg/m²)
- Power/mass: 0.098 hp/lb (160 W/kg)
See also 
- Related development
- Aircraft of comparable role, configuration and era
- Related lists
- "Boeing 307 Stratoliner entry at." The Aviation History Online Museum. Retrieved: January 28, 2012.
- Betts, Ed. "The Boeing Stratoliners and TWA." American Aviation Historical Society Journal, Volume 38, Issue 3, 1993.
- Hardy, Air International January 1994, p. 21.
- Bowers 1989, p. 231.
- Jane's Fighting Aircraft of World War II. London: Random House, 1989. ISBN 1-85170-199-0.
- Ford 2004, p. 55.
- Abzug, Malcolm J. and E. Eugene Larrabee. Airplane Stability and Control: A History of the Technologies that Made Aviation Possible. Cambridge, UK: Cambridge University Press, 2005. ISBN 978-0-521-02128-9.
- Hardy, Air International, January 1994, pp. 22–23.
- "Houston's Aviation History Timeline." The 1940 Air Terminal Museum. Retrieved: January 28, 2012.
- Munson 1972, p. 182.
- Taylor 1979, p.59.
- Hardy, Air International February 1994, p. 69.
- Bowers 1989, pp. 234–235.
- Hardy, Air International February 1994, p. 70.
- Hardy, Air International February 1994, pp. 70–72.
- Hardy, Air International February 1994, p. 71.
- Berry, Peter. "Transatlantic Flight 1938-1945 (Part I 1938-1943)." AAHS Journal, Volume 40, Issue 2, 1995.
- Berry, Peter. "Transatlantic Flight 1938-1945 (Part II 1943-1945)." AAHS Journal, Volume 40, Issue 3, 1995.
- Accident description for NX19901 at the Aviation Safety Network. Retrieved on 5 May 2013.
- Accident description for N75385 at the Aviation Safety Network. Retrieved on 5 May 2013.
- Accident description for F-BHHR at the Aviation Safety Network. Retrieved on 5 May 2013.
- Accident description for F-BELZ at the Aviation Safety Network. Retrieved on 5 May 2013.
- Accident description for 42-93030 at the Aviation Safety Network. Retrieved on 5 May 2013.
- Accident description for XW-TFR at the Aviation Safety Network. Retrieved on 5 May 2013.
- Accident description for XW-TFP at the Aviation Safety Network. Retrieved on 5 May 2013.
- "4 escape injury as historic Stratoliner ditches in Elliott Bay." Seattle Post-Intelligencer (original post). Retrieved: June 4, 2012.
- Whitford, Ellen. "Once More withe feeling." Boeing Frontiers Online, September 2003. Retrieved: January 28, 2012.
- "Green Design Will save the World: The Cosmic Muffin: A Boat Recycled From Howard Hughes’ Plane." Inhabitat. Retrieved: December 29, 2012.
- Bowers, Peter M. Boeing Aircraft since 1916. London: Putnam, Third edition, 1989. ISBN 0-85177-804-6.
- Bridgman, L. Jane's All the World's Aircraft 1942. London: Sampson Low, Marston & Co., 1942.
- Ford, Daniel. "First and Last 'Strat': Boeing's Model 307 and its Survivors". Air Enthusiast, No. 110, March/April 2004, pp. 54–60. Stamford, UK: Key Publishing. ISSN 0143-5450.
- Hardy, Mike. "The Stratoliner Story (Part 1)." Air International, Vol. 46, No 1, January 2004, pp. 21–24. Stamford, UK: Key Publishing. ISSN 0306-5634.
- Hardy, Mike. "The Stratoliner Story (Part 2)." Air International, Vol. 46, No 2, February 2004, pp. 69–72. Stamford, UK: Key Publishing. ISSN 0306-5634.
- Munson, Kenneth. Bombers in Service, Patrol and Transport Aircraft Since 1960. London: The Macmillan Co., 1972. ISBN 978-0-71370-586-7.
- Taylor, H.A. "Ten Big Boeings ... The Stratoliner Story". Air Enthusiast, Ten, July–September 1979, pp. 58–67. Bromley, UK: Fine Scroll.
|Wikimedia Commons has media related to: Boeing 307|
- Video Courtesy of *Bomberguy Aviation Historian
- Coast to Coast on Four Motors September 1940 Popular Mechanics
- History of the Boeing 307 by Boeing Historical Division