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Boeing 307 Stratoliner (C-75)

Four-Engine Passenger Airliner / VIP Transport Aircraft (1940)

Its trans-Atlantic capabilities saw the Boeing Model 307 pressed into service during World War 2 as the C-75 VIP transport.

Authored By: Staff Writer | Last Edited: 10/10/2017 | Content ©www.MilitaryFactory.com | The following text is exclusive to this site.

The B-17 "Flying Fortress" heavy bomber of World War 2 fame inspired Boeing to undertake a similar long-range, four-engined, civilian market passenger-hauler based on the successful components of the classic wartime aircraft. Work occurred in the mid-1930s and resulted in the company designation of "Model 307" to cover the design. The Model 307 was given an all new fuselage while retaining the wing mainplanes (along with their paired engines) and tail unit of the soon-to-be famous World War 2 bomber (in particular the B-17C production model).

In 1937, Pan American Airways (Pan Am) became the launch customer when it placed an order for two of the airliners and competitor TWA then followed with a five-strong order. Pan Am then ordered an additional four aircraft bringing the total of committed orders to eleven.

The aircraft featured a highly-streamlined fuselage with rounded nose section holding the cockpit. The cockpit was framed with rectangular-shaped windows for excellent views over-the-nose of the aircraft. The fuselage sides were dotted with rectangular windows for the passengers within and the wing mainplanes were fitted under the fuselage, each holding a pair of engine nacelles. The tail unit was traditional in arrangement with a single vertical fin and low-mounted horizontal planes while the undercarriage, made retractable, was of a tail-dragger design. The Model 307 was outfitted with 4 x Wright "Cyclone" GR-1820-G102 series air-cooled radial piston engines (each having single-speed superchargers) and these drove three-bladed propeller units. A crew of five would be used for optimal operation and passengers would sit in relative comfort on wide-body, high-back seats.

Boeing flew a prototype form for the first time on December 31st, 1938 though this specimen was lost in a crash occurring on March 18th, 1939 (after having shut down two of its engines in a test). After additional testing the Model 307 entered service with Pan Am on July 4th, 1940 as the "Stratoliner". Prior to this, millionaire playboy Howard Hughes received an early Model 307 example for his "round-the-world" record attempt but the arrival of World War 2 ended this endeavor. This example was then outfitted as a luxury flying apartment for the aviation trend-setter and pioneer.

After the Japanese surprise attack on Pearl Harbor, Hawaii on December 7th, 1941, the United States found itself committed to another World War courtesy of Europe. At this time, the United States Army Air Force (USAAF) required any and all aircraft for service to reach far-off places of the world. As such, many four-engined models were taken into service for the role and this included both land-based airplanes and flying boats - mainly to ferry high-ranking officials to-and-from European destinations and the American East Coast. The onset of war disrupted Boeing serial production of the civilian-minded Model 307 to the point that only ten were completed at the time.

At least five of the TWA Model 307 stock was adopted by the USAAF and these were modified by way of additional internal fuel tanks and sleeping berths with the removal of pressurization equipment as well as some passenger seating. The undercarriage was reinforced for the rigors of military service and, in this guise, the airliners came under the designation of "C-75" to reflect their cargo / transport roles.

Deliveries of these aircraft commenced in 1942 and the fleet served until mid-1944. Back in November of 1942, the military introduced a new, military-minded four-engined transport with cross-Atlantic capability as the C-54 "Skymaster" from Douglas Aircraft so this model began to take over the duties of the C-75s then in service. All C-75 flights were had over routes covering the Atlantic (the aircraft was not featured in the Pacific Theater).

As the war in Europe drew to a close during April-May 1945, the USAAF released its five C-75 transports back to TWA ownership. These were modified by Boeing back to their original civilian passenger-hauling forms and ended their days as such.

Variants in the Model 307 family included the Model 307B which increased the operating crew to seven and, perhaps more importantly, switched to the Wright Cyclone GR-1820-G105A series engines which supported two-speed turbochargers improving high-altitude flying. The Model 307B-1 was a post-war model given the wing mainplanes of the B-17G production bomber and updated electronics. The variant was powered by the Wright Cyclone GR-1820-G606 series air cooled radial engine.

Beyond their wartime service, the Model 307 saw use by several foreign operators (all civilian) including Cambodia, Ecuador, France and Laos. The Haitian Air Force operated the type briefly and the line was wholly retired in 1975 after decades of use.

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