Boeing 307 Stratoliner Pressurized Airliner

  • By Anthony E. Pomata
  • Posted 10/03/2001
  • Essay 3598
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Boeing’s little known 307 Stratoliner, affectionately dubbed "the flying whale" for its portly lines, ushered in a new aviation era when it entered into airline service in mid-1940. It was the first in-service pressurized airplane and airliner. It is cabin pressurization (termed cabin supercharging at the time), along with air conditioning and heating that enables today’s high altitude passenger jet airliner flights above the weather and turbulence, where the thin air and sub-zero cold could kill passengers within minutes were they unprotected. The Seattle-built, propeller driven Stratoliner took the first practical step on the journey to safe high altitude passenger flight. Although only 10 aircraft were built, it was very successful in airline service; one was reported still carrying passengers in 1986. Remarkably, at least two airframes survive today, the restored Pan American Airways NC19903 Clipper Flying Cloud, which began flying again on July 11, 2001, and the fuselage of the Howard Hughes special model, which is now a yacht. As luck would have it, the Flying Cloud was the first in-service pressurized airplane and airliner.

The Stratoliner was arguably the most advanced operational aircraft in the early 1940s, for it also utilized power boosted control surfaces and geared two-speed engine superchargers, lacking only the tricycle landing gear employed by the Douglas DC-5 airliner that entered service with Dutch airline KLM a month earlier. Building upon that lead, during World War II the U.S. was the only country to field:

  • a pressurized strategic bomber;
  • pressurized turbojet powered fighters;
  • a pressurized transport/airliner;
  • aircraft employing power boosted control surfaces.

Germany and the United Kingdom operationally deployed pressurized propeller-driven bombers and fighters, which were modifications of earlier aircraft. Today’s high performance turbine-engined civil and military aircraft are pressurized and utilize powered flight controls.

1930s High Altitude Flight

In the 1930’s aviation researchers realized that flying at high altitude above the weather would pay dividends in passenger comfort, higher speed, and longer range. Progress had been made to fly safely at high altitude: reliable oxygen masks, electrically heated flying suits, a practical pressure suit, and a successful experimental pressurized airplane that flew in May 1937. During this period, airlines, the military, and individuals conducted high altitude flight tests, which resulted in several U.S. and a British airline requesting proposals for pressurized airliners.

Boeing, Curtiss, and Douglas responded with designs, all of which reached the flying hardware stage by 1940. Britain’s Fairey built a mockup before the project was cancelled in 1939 due to World War II. First in the air, however, was the U.S. private venture Abrams Explorer two-crew photomapping airplane flown during November 1937. The Explorer, the only one built, flew successfully for many years and now resides in the National Air & Space Museum collection in Washington D.C.

Stratoliner Derived from B-17 Bomber

Wellwood Beall, famed 314 Clipper flying boat designer, led a talented team that in December 1935 began development of the 307 as an airliner derivative of the model 299/XB-17 Flying Fortress. Douglas, by 1936, had five airlines sponsoring development of its pressurized DC-4E four-engine long-range landplane airliner. Pan American Airways (PAA) and Trans Continental and Western Airlines (TWA) decided before it flew that they wanted out, due to high costs and projected performance shortfalls. In 1937 they ordered instead the 307, four for PAA, five for TWA. Millionaire Howard Hughes later ordered another. These were to be the total orders for the aircraft, which cost approximately $250,000 delivered. Breda of Italy had sought a production license in 1939, wanting the 307 for transatlantic service and for its technology. Political and Not Invented Here considerations evidently killed the project.

First Flight in 1938

Before the Stratoliner took wing, a confident Boeing designed a huge pressurized two-deck flying boat in response to a 1937 PAA requirement for a flying ocean liner capable of crossing the Atlantic non-stop. Boeing’s model 326 was headline news on June 22, 1938, its announcement coming just 15 days after the 314 Clipper flew. However, Boeing built none of the model 326. Nor were any of four competing designs ever built.

The S-307 NX19901 prototype (for PAA) flew for the first time on December 31, 1938, piloted by Eddie Allen, from Boeing Field, Seattle, for a total of 42 minutes. The first pressurized flight, successfully accomplished by PAA NC19902 Clipper Rainbow, occurred on June 20, 1939.

Design Relationships and Large Vertical Tails

Initially the 307 design was based on the model 299/XB-17 -- wings (over three feet wider), tail and landing gear joined to a new circular section pressurized fuselage with newer B-17B type engines without turbo-superchargers. Production aircraft had wing slots, a dorsal fin, and a large vertical fin -- these last two items were then developed for the B-17E through G models. Large vertical tails have since characterized Boeing airplanes up to the present 777 jet airliner. Development of the B-29 Superfortress pressurization system stemmed from the 307, and this successful bomber was the first mass production pressurized airplane.

Stratoliner Name

The Stratoliner was the first of several Boeing aircraft to use the strato prefix in its name. Strato is derived from the second-from-the-surface of the earth’s atmospheric layers, the stratosphere, which begins at around 30,000 feet of altitude. TWA highflying model SA-307B’s were shy by about 4,000 feet of being able to reach that height. Some early 707 turbojet airliner models were for a time also named Stratoliner -- they cruised comfortably in the stratosphere.

Stratoliner Entered Service in 1940

PAA’s Flying Cloud flew the first operational pressurized service from Miami, Florida, to Latin America beginning on July 4, 1940. The Stratoliner offered unmatched comfort, speed and range advantages over its Douglas DC-3 and Lockheed Electra twin-engine competitors. A wide body airliner, its fuselage was more than three feet wider than the DC-3, and featured a luxurious 33-passenger cabin -- pressurized, air conditioned and heated; passenger compartments; sleeping berths with windows; ample-size individual reclining sleeper seats; large seat windows (approx. 12 x 16 inches); men’s and women’s lavatories with skylights; and a galley with a skylight.

A month later, a German Junkers Ju 86P two-crew high altitude photo aircraft flew at 41,000 feet over the United Kingdom, becoming the first operational pressurized military airplane.

Boeing Was the 1940s Technology Leader

In 1940 Boeing was king of the hill in advanced technology with its stable of operational airplanes, while Douglas and Lockheed led in sales. With a six-month lead on the 307, the 314 Clipper flew on June 7, 1938. In service with PAA, it was at once the largest, heaviest, longest-range, highest capacity airliner, using the most powerful engines. In the 307 Boeing had the only operational pressurized airplane and the longest-range landplane airliner. Despite having the longer range 314 in service, PAA briefly considered flying the 307 across the north Atlantic, but never did.

During World War II, the Army Air Force (AAF) flew the route with its impressed TWA 307’s beginning in 1942. Boeing’s B-17C was the fastest, highest flying, and longest-range heavy bomber in the air. Adding to this bounty was the XB-29 then being developed, among the most advanced operational airplanes of World War II, with reversible pitch propellers, tricycle landing gear, electronic computer controlled powered gun turrets, and navigation/bombing/tail warning radar systems.

A Special Model for Howard Hughes

A special model SB-307B for Howard Hughes was built with more powerful engines and extra fuel tanks for an around-the-world flight that was cancelled due to the start of World War II. The flight was never made. It was the first Stratoliner delivered to a customer; its initial flight (with experimental license NX19904) occurred on July 13, 1939. Postwar it was fitted with a luxury interior, including a bedroom, and named The Flying Penthouse.

A 1964 hurricane severely damaged it and rendered it unflyable. In 1969 it was purchased as scrap for $61.99 -- the fuselage was salvaged (the aft rounded pressure bulkhead formed the cabin after end), then mounted on a boat hull and converted into a luxury yacht named The Londonaire. It was rebuilt beginning in 1994, and is a Florida based, operating yacht named Cosmic Muffin, with N19904 painted on its sides.

World War II Service and Afterwards

During World War II, TWA’s 307’s were taken into AAF service as the camouflaged C-75. PAA aircraft were retained by the airline, with their crews and colors flying under charter for the AAF Air Transport Command. All eight aircraft survived their wartime service.

Postwar Stratoliner airline service began in late 1945 when TWA resumed coast-to-coast flights with its upgraded SA-307B-1 aircraft, and PAA flew the New York City to Bermuda route. PAA Stratoliner service ended in 1948, when its three aircraft were sold. TWA used its five SA-307B-1’s until 1951, after which they were sold off. Three aircraft were in service in Indochina during the 1970s, one aircraft was reported still flying in Laos as late as 1986.

Restored Flying Cloud Airborne

Resplendent in its highly polished aluminum Art Deco finish, the Flying Cloud flew from Boeing Field exactly 61 years and one week after it flew into the history books. The July 11, 2001, flight was actually its third first flight. It flew for the first time in 1939 from Boeing Field. During its active years, the Flying Cloud served not only PAA, but also the President of Haiti and several other owners in its productive lifetime. It made a second first flight on June 4, 1994, to Boeing Field after languishing in the open in the Arizona desert for more than 22 years. Over a seven-year period, a team of Boeing employees and volunteers refurbished it in its Plant II birthplace. Owned by the National Air & Space Museum, the Flying Cloud will be permanently displayed in the museum’s Washington D. C. facility.

[Note: On March 28, 2002, the restored Stratoliner developed engine trouble while on a test flight and ditched into Elliott Bay. No one was injured, and the damaged aircraft was retrieved. The Flying Cloud evidently ran out of fuel, causing it to descend into the water. As of June 2002, Boeing has determined that the necessary repairs to restore the Flying Cloud to flyable condition are cost effective. A team of Boeing and volunteer workers will rebuild it, with the intention of flying east in the summer of 2003 to the Smithsonian Air and Space Museum.]

Stratoliner Facts

  • First operational airplane with hydraulically boosted control surfaces -- elevators and rudder;
  • Fastest scheduled long range airline service -- up to 220 mph cruise, flown by TWA model SA-307Bs beginning in 1940;
  • First airliner (SA-307B) with geared two speed engine superchargers able to cruise at high altitude with passengers in complete comfort beginning in 1940;
  • First four-engine landplane airliner in U.S. scheduled long-range service;
  • Wide body fuselage -- wider at 138 inches/11.5 feet overall than its turbojet powered namesake, the 367-80 Dash Eighty 707 prototype tanker/airliner at 132 inches/11.0 feet.


Boeing Archives; Boeing S-307 Maintenance Manual (Seattle, 1940); S-307 Clipper Flying Cloud restoration project in Boeing’s plant 2: Einar Moen/leadman, Ed Heineman/volunteer; PAA newspaper, March 1940; Bob Stubbs/PAA flight engineer; Ralph Connerly/PAA S-307 1940s radio operator; Dave Drimmer/owner of the yacht Cosmic Muffin; The Seattle Daily Times, June 23, 1938; Kenneth Munson, World Aircraft From 1919 to 1935 (London, U.K.: Blandford Press Ltd., 1982); Airliners From 1919 To The Present Day (London, U.K.: Blandford Press Ltd., 1982); Bill Gunston, Combat Aircraft of World War II (London, U.K.: Salamander Books Ltd., 1978); Giorgio Bignozzi, "The Italian Fortress," Air International, December 1986; Joachim Dressel and Manfried Griehl, Junkers Ju 86 (Atglen, PA: Schiffer Pub. Ltd., 1998); Clarence L. Johnson, Development of the Lockheed Constellation (Burbank, CA: Lockheed Corp., 1944); Air Transport, May 1945; Aviation Magazine (1923-1940); The New 1000 H.P. Wright Cyclone (Patterson, NJ: Wright Aero. Corp., 1936?); Pratt and Whitney Engine Handbook (Hartford, CT: Pratt and Whitney Aircraft Co., November 1927); Pratt & Whitney Operators Handbook; Wasp and Hornet, Wasp Junior (Hartford, CT: Pratt and Whitney Aircraft Co., March 1, 1930); "Man's Farthest Aloft," National Geographic, January 1936; David M. Carpenter, Flame Powered: The Bell XP-59A Airacomet and the General Electric I-A Engine (New York, NY: Jet Pioneers of America, 1992); Stanley R. Mohler and Bobby H. Johnson, Wiley Post, His Winnie Mae, and the World's First Pressure Suit (Washington D.C.: Smithsonian Institution Press, 1971); H. A. Taylor, Fairey Aircraft Since 1915 (London, U.K.: Blandford Press Ltd., 1988); Graham White, Allied Aircraft Piston Engines of World War II (Warrendale, PA: Society of Automotive Engineers, Inc., 1995); Frank L. Greene, The Grumman F4F-3 Wildcat, Profile No. 53 (Surrey, U.K.: Profile Pub. Ltd., 1965); Richard Thruelsen, The Grumman Story (New York, NY: Praeger Publishers, Inc., 1976); The Boeing Company Website (; (; National Air and Space Museum (; Boeing/Douglas Division Archives; Bobbi Trout (record setting aviatrix who conceived and tested the first successful electrically heated flying suit); Los Angeles County Natural History Museum; Seattle Museum of Flight Technical Library; National Air & Space Museum aeronautics department; Boeing Public Relations/jet airliner pressurization data - via Bill Seil; Author's 1998 volunteer work on the Flying Cloud restoration project and visit to the yacht Cosmic Muffin.

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