Boeing 307 Stratoliner

The Boeing Model 307 Stratoliner was an American commercial transport aircraft that entered commercial service in July 1940. 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). 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 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) prompted Boeing to begin production on an initial batch of the airliner. Pan Am received its initial order and TWA received its order, but only one of the second batch of four Pan Am aircraft was delivered before war intervened and put a halt to civil aircraft production.
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 January 26, 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 fuel tanks. The landing gear was strengthened, the maximum takeoff weight was increased from , and the exterior was 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.
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 (NX-19904) 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 the luxurious "The 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 (NC-19902:"Clipper Rainbow", NC-19903:"Clipper Flying Cloud", and NC-19910:"Clipper Comet") and five to TWA (NC-19905:"Comanche", NC-19906:"Cherokee", NC-19907:"Zuni", NC-19908:"Apache", and NC-19909:"Navajo") with one aircraft going to Hughes (NX-19904).
On the entry of the United States into World War II, Pan Am (PAA) continued operating its Stratoliners on routes to Central and South America, but under direction of the Army Air Forces' Air Transport Command, while the TWA 307s were sold to the United States government, being designated Boeing C-75 (42-88623/27) 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, which 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.
Pan Am flew its unmodified 33-passenger Stratoliners between Miami and Havana until 1947, then sold them to small operators, including Aerovias Ecuatorianas AREA). Some later returned to the US, whilst others went to South-East Asia to replace write-offs. One aircraft (N-19903) was purchased by the Haitian Air Force, being fitted as a Presidential transport for François "Papa Doc" Duvalier. This aircraft later returned to the U.S. and was purchased by the Smithsonian Museum.
TWA sold its Stratoliner fleet, of five, to the French airline Aigle Azur, which used them (F-BELU, F-BELV, F-BELX, F-BELY, F-BELZ) on scheduled flights from France to North and Central Africa, and later to French Indo-China. Three aircraft were transferred (F-BELU, F-BELV, F-BELX) to Aigle Azur's Vietnamese subsidiary (Aigle Azur Extrême-Orient) and were used in South East Asia, in various liveries and with various registrations, with at least one aircraft remaining in commercial use until 1974. Two aircraft remained in Europe (F-BELY, F-BELZ) and passed to Mediterranean runs, especially around Corsica, with: Airnautic. The exact history of the five aircraft becomes quite complex as the subsidiary was sold to UAT in 1955, and this in turn merged to become UTA in 1963, whilst the original branding was kept until 1961. Also, some of these aircraft were under contract to the ICC/CIC [see below], running both ad hoc flights plus regular scheduled services for diplomats between the various, often hostile, capitals. Aircraft write-offs required buying-up some of the remaining 307s, for ease of maintenance and continuity of service, so F-BELY also went East, as did the ex-Pan Am F-BHHR. At various times following independence, most received local (usually Lao) registrations (XW-PGR; XW-TAA, XW-TAB, XW-TAC; XW-tFP, XW-TFR), possibly for legal/fiscal reasons, whilst maintaining essentially the same services. Some were registered to the CITCA leasing company, whilst some formed the basis of the national-carrier Royal Air Lao. 
C-75.
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.
Variants.
Airframes.
Only 10 airframes were ever built:
Accidents and incidents.
According to the Aviation Safety Network, the Boeing 307 was involved in eight hull-loss incidents for a total of 67 fatalities. Four of the eight incidents were fatal, with one an apparent shoot-down in a zone of conflict; casualties, by number, in the four incidents were: 25, 19, 13 & 10 (total: 67).
Surviving aircraft.
The only surviving Boeing 307 Stratoliner (NC-19903) is preserved in flying condition at the Smithsonian Museum's Steven F. Udvar-Hazy Center. On March 28, 2002, this aircraft crashed 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.
The fuselage of Howard Hughes' personal 307 (NX-19904) also survives, although it has been converted into a houseboat. The aircraft was awaiting restoration at Fort Lauderdale International Airport in the early 1960s when it was severely damaged in a hurricane after its tiedowns failed and it was blown into a stand of trees. The aircraft languished for nearly a year before being removed and longer still until later salvaged for its conversion into the house boat. The interior is notable for the original finishes and fitments added by Howard Hughes. The boat is now part of the collection at the Florida Air Museum.