The world’s first pressurized airliner took passenger transport to new heights.
Anyone who has admired the polished aluminum perfection of the Pan American Airways Boeing 307 Udvar-Hazy Center would reject its early nickname, the “Flying Whale.” What’s more, the Stratoliner’s Flying Cloud at the Smithsonian’s beauty was never just skin-deep: The largest land-based airliner developed during the 1930s, it established an impressive new standard for passenger comfort. Unlike the ubiquitous Douglas DC-3, though, only a handful of Stratoliners actually entered service. Boeing’s big transport nonetheless advanced aeronautics far more than was appreciated in its day.
Some historians maintain that Boeing was already considering a design similar to the 307 when William John “Jack” Frye approached the company with the idea in the 1930s. But the Stratoliner concept can arguably be traced to the vision of Frye, a pilot who became president of the airline that evolved after the 1930 merger of Transcontinental Air Transport and Western Air Express to become Transcontinental and Western Air (TWA). What is certain is that Pan American’s Andre Priester and Charles Lindbergh had also become interested in developing a large transport aircraft capable of flying above the weather.
Dependable twin-engine Douglas DC-2s and -3s were beginning to firm up the fractured airline industry in the mid-’30s, leading some airline executives to believe that twin-engines were the ticket to growth. But eyeing the progress being made on the four-engine Army Air Corps Boeing B-17 and Consolidated B-24 long-range bombers then under development, Frye signed a nearly $2 million contract with Boeing on January 1, 1937, for six four-engine transports. Pan Am followed with an order for four planes.
The 307 would be a 42,000-pound hybrid derived from the B-17C, with similar wings, tail, nacelles and engines. Its fat fuselage featured a circular cross section that distributed the stresses of pressurized air equally throughout the plane’s structure, enabling the 307 to maintain a constant cabin pressure equivalent to an altitude of 8,000 feet while cruising at 20,000 feet. The airliner also had hydraulically boosted control surfaces, the first on an operational aircraft, and geared two-speed engine superchargers. Construction on the new transport began in the summer of 1937.
Dubbed Stratoliners, the 307s were originally designed to carry 33 passengers in daytime configuration and 25 at night, with a crew of five. Two flight hostesses (actually registered nurses) tended to passengers. The new airliners offered a significant advantage over twin-engine transports: Thanks to cabin pressurization, passengers could count on a much smoother and more comfortable trip, since they’d be flying above the weather in cabins that boasted reclining seats, air-conditioning and heating, not to mention a galley and men’s and women’s skylighted lavatories. Stratoliners also became the first land-based four-engine airliners to employ a flight engineer, who was responsible for monitoring power settings, cabin pressurization and other subsystems.
TWA’s interiors were created by well-known industrial designer Raymond Loewy, and fitted out with furnishings from the upscale retailer Marshall Fields. On daytime flights passengers had access to a chaise lounge and dressing rooms. A sleeper version offered 16 berths and nine chaise lounges. On the whole, the 307’s accommodations exceeded those of the plush Boeing, Martin and Sikorsky flying boats then crossing the oceans.
Manufacturing problems associated with the pressurization system created difficulties for Boeing, but they were overcome and production continued. Boeing’s Eddie Allen took the controls on December 31, 1938, for the 307’s maiden flight, subsequently giving the new transport a thumbs up.
On March 18, 1939, Julius Barr piloted a prototype Stratoliner destined for Pan Am during a routine test flight at Alder, Wash., near Mount Rainier. It crashed, killing all aboard, including representatives of KLM Royal Dutch Airlines. Barr was reportedly testing the 307’s stability with one engine shut down at the time, and the cabin pressurization system had not been operating. Since Pan Am declined to increase its order, only three 307s would be built for that airline.
Several months later, following extensive wind tunnel tests, engineers determined the Stratoliner design was aerodynamically flawed, with ailerons and rudder that were too small to allow for recovery from a stall and spin when the plane flew at a slow speed with two engines shut down on one side. Major design changes were made to increase stability, including adding a dorsal fin to the vertical stabilizer and slotted leading edges on the wings, as well as enlarging the rudder.
On another proving flight between Kansas City and Albuquerque in May 1940, a 307 slated for TWA was being tested at 20,000 feet with cabin pressurization when it ran into a violent thunderstorm and all four engine carburetors iced up. The stricken plane barely recovered at treetop level, and the pilot managed to land it on a small plateau. With its wheels only partially down, it skidded to a belly landing, stopping just a few feet from a deep ravine. None of the 14 occupants was injured, and the plane was later flown out after minor repairs. Strangely, the mishap was hardly mentioned by the press.
Pan Am’s three “Strato-Clippers,” christened Rainbow, Comet and Flying Cloud, received their Civil Aeronautics Authority certification in March 1940. Regular service began on July 4 from Brownsville, Texas, to Mexico City and Los Angeles.
TWA put its first SA-307B into service on July 8 on the transcontinental run from LaGuardia Airport to Burbank Airport, making the flight in 14 hours and nine minutes with stops in Chicago, Kansas City and Albuquerque—nearly four hours less than the DC-3 record. TWA gave the 307s in its fleet Indian tribal names: Apache, Cherokee, Comanche, Navaho and Zuni.
On December 13, 1941, President Franklin D. Roosevelt took the first step toward mobilizing airlines’ resources after the Japanese attack on Pearl Harbor when he directed Secretary of War Henry L. Stimson to take control of any civil airline assets necessary for the operation of the military air transport system. At the end of December, TWA’s five Stratoliners were transferred to the government for long-range operations as combination passenger/cargo carriers. Converted to U.S. Army Air Forces specifications, they were designated C-75s. The planes’ heavy pressurization equipment was removed, extra fuel tanks were added and olive-drab camouflage replaced the airline markings. Maximum gross weight rose from 44,000 to 55,000 pounds.
Maintained and operated by TWA personnel under Air Transport Command’s new Intercontinental Division, the C-75s began flying to war zones across the North and South Atlantic as required, independent of any domestic transcontinental operations. They were manned by the airline’s senior pilots and flight engineers, wearing the same uniforms as Army Air Forces personnel but with civilian insignia. The early transatlantic flights were all pioneering efforts for the TWA crews, since the C-75s were the first land transports to operate between the United States and the European theater. While at first they flew on an unscheduled basis, regularly scheduled flights soon commenced. Original destinations included Scotland and England, typically via Newfoundland and Greenland, or the Azores to North African bases and on to India and China. Ascension Island was the usual stop for flights between Brazil and North Africa.
Three of the five C-75s were allocated to transatlantic service and the other two to the Washington-Cairo route. The latter flights, many of which took off from Washington’s National Airport, often lasted 20 hours. A TWA C-75 made a survey flight from Bolling Field in Washington to Cairo on February 26, 1942.
Atlantic crossings were usually scheduled at night, so it would be easier for navigators to reference stars and the moon for sextant sightings. Pilots were warned to avoid flying near convoys during visual flight rules conditions because Navy ships had been ordered to shoot down any aircraft overhead. The passengers were mostly high-ranking military or government officials; any cargo had to have a very high priority. Regular military flights for VIPs to Britain began in April 1942.
Unlike TWA, Pan Am did not officially transfer ownership of its three Stratoliners to the government, and consequently they did not get the militarized C-75 designation, although Air Transport Command directed most of their flights. Reconfigured to accommodate 45 passengers, the airliners were not otherwise modified, continuing to fly in Pan Am colors. They also retained their pressurization equipment, thus becoming the first pressurized aircraft to serve as military transports. Pan Am’s 307s operated on routes to Central and South America from Miami, New Orleans and Brownsville.
At the time airline pilots assigned to domestic routes were limited to flying 100 hours per month, but no limits had been set for those flying international routes. One TWA captain flew 271 hours over 39 days, including two round trips between Washington and Cairo.
Because of the 307’s plush accommodations, many celebrities had been 307 passengers on coast-to-coast flights before the war. Movie stars, USO entertainers, ambassadors, generals, admirals and leaders such as President Roosevelt, Chiang Kai-shek and Winston Churchill all took international flights aboard Stratoliners early in the war.
By early 1944, however, when more Douglas DC-4/C-54s became available, the U.S. government began returning TWA’s Stratoliners to the company. The last of them was released to the airline in late April 1944. Three of the C-75s, Apache, Comanche and Zuni, had made a total of 120 transatlantic crossings.
TWA’s five Stratoliners compiled a nearly perfect safety record during the war. The only mishap was a belly landing at Burbank, Calif., after a C-75’s main gear wouldn’t fully extend. There were no injuries in that accident. Another C-75 had a close call during a flight between Reykjavik, Iceland, and Gander, Newfoundland. Cruising at about 1,000 feet at night to avoid turbulence, the plane came too close to a convoy and was fired on (it had apparently been mistaken for a Focke Wulf Fw-200 Condor). An anti-aircraft artillery shell hit its tail section, nearly severing the control cables. But five harrowing hours later the crippled C-75 arrived safely at its destination.
Boeing modified the five C-75s to comply with Civil Aeronautics Administration licensing requirements and returned them to TWA intercontinental service as SA-307B-1s. The interiors were revamped with soundproofing, new galleys and improved cabin and cockpit temperature controls.
TWA pilots generally gave the Stratoliner high marks. However, Captain Bob Buck, a company test pilot, reported in his memoirs: “It was a little squirrelly on the ground, and the cockpit visibility was bad. From the outside, the cockpit looked like a greenhouse with its wrap-around windows, but this was misleading. The pilot seats were so far back that the windshield seemed to be at the end of a tunnel.”
According to Robert J. Serling, author of Howard Hughes’ Airline, a TWA company history: “The Stratoliner’s transcontinental flights were about two hours faster than the DC-3’s and the difference would have been even greater if TWA had operated them at higher altitudes. Most all 307 trips were flown at 14,000 feet; TWA was playing it conservatively. Even so, the Stratoliner won instant popularity because 14,000 feet was sufficient to clear most of the weather.” He added, “TWA’s pilots admired their new big bird immensely, and the veterans still remember it with respect.”
The DC-4 Skymaster and Lockheed 1049 Constellation eventually outclassed the Stratoliner, and TWA sold its five 307s to Aigle Azur Transports, a French company, in 1951. The French operated them out of Laos during the French-Indochina War as well as the Vietnam War. In 1959 four of the Stratoliners returned to Europe, subsequently flying cargo between France and Germany.
On December 29, 1962, Navajo was involved in a fatal accident on the island of Corsica. The remaining three planes were transferred to another French company. Comanche disappeared with 12 people aboard on a flight from Laos to Hanoi on October 18, 1965. Zuni is believed to have crashed, killing two of its four crewmen and 14 of its 21 passengers, on June 27, 1974. Apache was last known to be operated by a charter carrier in Cambodia in 1974. Cherokee, the last of the TWA Stratoliners, was reportedly flying for Air Vientiane in Laos in 1986, but its fate is unknown.
Pan Am’s three Stratoliners were released to the company in 1946, making daily flights between New York and Bermuda until they were sold in 1948. They had numerous owners during their remaining lifetimes. Comet crashed near Madras, Ore., while crop-dusting in 1958; Rainbow crashed during a landing at Saigon’s Tan Son Nhut Airport in May 1961 and was scrapped for spare parts.
The Airline Training Co. of Florida purchased the last of Pan Am’s 307s, Flying Cloud, in 1948. Acquired by the Haitian air force in 1954, it was used for a time as President François “Papa Doc” Duvalier’s personal plane. In 1957 it returned to the United States, where it was bought by the Flight Investment Co. of Dallas in 1959. After changing hands several more times, it was sold to a company in Mesa, Ariz., and parked at Falcon Field until 1973. It was about to be transformed into a firefighter when Robert C. Mikesh, a National Air and Space Museum curator, intervened. Realizing the aircraft’s historical significance, Mikesh arranged to have it traded for a C-121 Constellation. Permission was granted for a one-time flight to Davis-Monthan Air Force Base, and the 307 was then towed to nearby Pima County Air Museum, where it remained until 1979.
Boeing employees noticed the 307 there when they began working on the restoration of the Boeing 367-80, the first American jet transport—better known as the “Dash 80”—which was already slated for display at the Smithsonian. They persuaded the company to have Flying Cloud put in flyable condition again and flown to Seattle in June 1994, where a group of about 30 dedicated volunteers, including retired airline pilots, embarked on a multiyear restoration process. After a rollout ceremony in the summer of 2001, Flying Cloud made the round trip from Seattle to Oshkosh, Wis., for that year’s EAA AirVenture.
More work on the interior was still needed before the restoration would be complete. A plan developed to have the last of the Stratoliners make its final flight from Seattle to Washington’s Dulles International Airport, adjacent to the new Udvar-Hazy Center, then under construction. Following several local test flights, on March 28, 2002, the 307 flew to Paine Field, about 25 miles from Seattle, with a crew of four so that two pilots could practice landing and also refuel. The crew conducted one full-stop landing, then taxied back for another takeoff. During that second takeoff one engine experienced a momentary overspeed (turning beyond its design limit), and the crew decided to return to Seattle without refueling. On the approach for landing, the left main gear would not extend, so the flight engineer went below and manually cranked it down while they circled. When they were about six miles from the airport, the low fuel pressure light on one of the engines came on. The pilot feathered the propeller, but the other three engines also lost power. He opted to ditch close to the shore in nearby Elliott Bay, in 30 feet of water.
The four-man crew escaped without injury, but there was substantial damage to the landing gear and engine mounts, as well as to the plane’s interior. The engines had quit due to fuel exhaustion, and the National Transportation Safety Board said the probable cause of the accident was “the flight crew’s failure to accurately determine onboard fuel during the pre-flight inspection.” A contributing factor was “a lack of crew communication regarding the fuel status.”
After evaluating the waterlogged Stratoliner, Boeing committed to restoring the 307 to flight-worthy condition once again for the 2,600- mile flight to Washington. Rolled out on June 13, 2003, Flying Cloud flew across the country and made its last landing at Dulles, near where it can be seen on display today. That August, the Dash 80 also arrived at Dulles, and now stands proudly near the gleaming 307, its fellow history-maker.
Contributing editor C.V. Glines is the award-winning author of numerous aviation books and articles. For additional reading, he recommends Legend and Legacy: The Story of Boeing and Its People, by Robert J. Serling.
Originally published in the January 2009 issue of Aviation History. To subscribe, click here.