The Boeing Model 307 Stratoliner was the first commercial transport aircraft with a pressurized cabin. This feature allowed the plane to cruise at an altitude of 20,000 ft (6,000 m), well above 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 aircraft to include a flight engineer as a crew member.
The maiden flight of the first Boeing 307 (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, breaking up during spin recovery.
The first delivery to a customer was to multi-millionaire Howard Hughes, who purchased one to carry out 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 307 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. It 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, being sold to oil tycoon Glenn McCarthy in 1949.
Deliveries to Pan-Am started in March 1940, with TWA receiving its first aircraft in April. TWA's aircraft were used on services between Los Angeles and New York, while Pan-Am's aircraft flew routes down to Latin America. In all, only 10 aircraft were built, with three being delivered to Pan-Am and five to TWA, together with the one aircraft for Howard Hughes. 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 aircraft were sold to the US Government, being designated Boeing C-75 and operated by the United States Army Air Forces (although normally still flown by TWA crews).