On April 7, 1924, the U.S. Post Office requested proposals for a mail service aircraft to replace its current fleet of aircraft, including the Curtiss JN-4H and de Havilland DH-4, which had been converted from their military configurations. Boeing, along with 11 other aircraft manufacturers, submitted designs on July 15, 1925. Boeing's submission, the Model 40 biplane, featured a wood frame fuselage and wood and fabric wings. It was powered by the 12-cylinder, 400-horsepower, water-cooled Liberty engine, as required by the government in an effort to utilize surplus equipment from the war. Boeing was unsuccessful, with the contract being awarded to Douglas.
In early 1927, the U.S. Post Office began turning over airmail service to private enterprises and, once again, requested proposals for several routes. Boeing dusted off the Model 40 and set about redesigning a winning aircraft. The result was the Model 40A. This version featured welded-steel tubing in the fuselage and a new powerplant, the 420-horsepower, air-cooled Pratt & Whitney Wasp engine. The steel-tube framing reduced weight and operating costs, while increasing the useful load of the aircraft. This improvement, combined with the lighter and more reliable Wasp engine, helped Boeing Air Transport win the mail service contract.
On July 1, 1927, the first day of Boeing's new airmail contract, 24 Boeing Model 40As entered service along the coveted Contract Airmail Route (CAM) 18 that connected San Francisco and Chicago. In addition to its ability to carry 1,200 pounds of mail, the Model 40A also had room for two passengers, becoming the first Boeing aircraft to carry commercial passengers. The Model 40B would become the primary production model, incorporating a larger Pratt & Whitney R-1690 Hornet engine. The Model 40B could carry four passengers and 500 pounds of mail. In total, 77 Model 40s, including all variations, were produced between 1925 and 1932.
The Boeing Model 40A featured a wingspan of 44.17 feet and a length of 33.17 feet. The aircraft's gross weight was 6,000 pounds, providing 2,469 pounds of useful load. The Model 40A had a top speed of 128 miles per hour, with a cruise speed of 105 miles per hour. The aircraft had a range of 650 miles and could operate up to a maximum ceiling of 14,500 feet, although mail routes were usually flown in close proximity to the ground.
