The Boeing Model 40 is a United States mail plane of the 1920s. It was a single-engined biplane that was widely used for airmail services in the United States in the 1920s and 1930s, especially by airlines that later became part of United Airlines. It became the first aircraft built by the Boeing company to carry passengers. In 1925, the US Post Office issued a requirement for a mailplane to replace the ex-military DH-4s then in use. The new aircraft was required to use the same water-cooled Liberty V12 engine as used by the DH-4, of which large stocks of war-built engines were available. The resultant aircraft, the Boeing Model 40, was a conventional tractor biplane, with the required Liberty engine housed in a streamlined cowling with an underslung radiator. The aircraft's fuselage had a steel tube structure, with an aluminum and laminated wood covering. Up to 1,000 lb (450 kg) of mail was carried in two compartments in the forward fuselage, while the single pilot sat in an open cockpit in the rear fuselage. The wings and tail were of wooden construction, and the Model 40 had a fixed conventional landing gear. The Model 40 made its first flight on July 7, 1925. Although the prototype was purchased by the US Post Office, the production order went to the Douglas M-2. The Contract Air Mail Act of 1925 set out the gradual privatization of the Post Office's Air Mail routes. In late 1926, bids were requested for the main transcontinental trunk mail route, which was to be split into eastern and western sections, with Boeing bidding for the western section. Boeing revived the design for the tender, with the Model 40A replacing the Liberty engine with a 425 hp (317 kW) air-cooled Pratt & Whitney Wasp radial engine, which was 200 lb (91 kg) lighter than the Liberty, even ignoring the weight of the Liberty's radiator and cooling water. The fuselage was redesigned to make more extensive use of welded steel tubing, and an enclosed cabin was fitted between the mail compartments, allowing two passengers to be carried as well as 1,200 lb (540 kg) of mail. Boeing's bid of $3 per lb was much less than any of the competing bids, and Boeing was awarded the San Francisco to Chicago contract in January 1927, building 24 Model 40As for the route (with a further aircraft being used as a testbed by Pratt & Whitney). The next model to reach production was the Model 40C, with an enlarged cabin allowing four passengers to be carried. Meanwhile, Boeing Air Transport's Model 40As were modified by replacing their Wasp engines with 525 hp (391 kW) Pratt & Whitney Hornet radial engines to become the Model 40B-2. The Model 40B-4 was a new-build aircraft combining the four-passenger cabin of the Model 40C with the Hornet engine of the B-2. Production continued until February 1932. Boeing's airline, Boeing Air Transport, commenced operations on the San Francisco–Chicago route on July 1, 1927. Variants Model 40 Original 1925 design with Liberty engine. Model 40A Revised 1927 design for BATC. the aircraft was powered by a Pratt & Whitney Wasp radial engine, plus seating for two passengers in an enclosed cabin; 25 built. Received Dept of Air Commerce Approved Type Certificate #2. Model 40B Model 40As re-engined with a 525 hp (391 kW) Pratt & Whitney Hornet radial piston engine. 19 Model 40A were converted. Redesignated Model 40B-2. Model 40B-4 Revised Model 40B with seating for four passengers and other improvements. Equipped with openable windows, plus seating for four passengers; 38 built. Model 40B-4A (Specifications below) One Model 40B used as engine testbed by Pratt & Whitney. Model 40H-4 Four Model 40B-4s built by Boeing Canada. Two aircraft were exported to New Zealand. Model 40C Similar to Model 40B-4 but with Pratt & Whitney Wasp engine of Model 40A. (ten built, all later converted to Model 40B-4 standard). Model 40X Unique special-order machine similar to Model 40C with only two-passenger cabin and extra open cockpit forward of pilot's cockpit. Model 40Y Unique special-order machine similar to Model 40X, but with Pratt & Whitney Hornet engine.
