Boeing Model 40

To meet the requirements of the Air Mail Department of the US Post Office, which needed a new mailplane to replace its ageing DH-4s, Boeing designed in 1925 a large biplane transport under the designation Boeing Model 40. It was required to compete against the submissions of other manufacturers, and the Post Office specification had stipulated the use of a Liberty engine, plus an ability to carry 454kg of air mail. A fairly conventional biplane of that period, with tailskid landing gear and a Liberty engine, the Model 40 had a mail . compartment in the forward fuselage, with the pilot seated well aft in an open cockpit. First flown on 7 July 1925, the Model 40 was unsuccessful in the competition, the Douglas entry being declared winner. This was to result in Boeing's design gathering factory dust for some 18 months until, in early 1927, the US Post Office began the process of turning the government air mail service over to private enterprise.

Requiring an aircraft to operate on any of the routes for which it might bid, Boeing dusted off the Model 40, and began a process of redesign and conversion to make it suitable for the sort of operation the company had in mind. The resulting Model 40A had three major changes by comparison with the original Model 40 prototype: the Liberty engine was replaced by a Pratt & Whitney Wasp radial; its composite structure fuselage gave place to one of steel tube with fabric covering; and better use was made of fuselage capacity. The pilot's position remained unchanged, but an enclosed cabin for two passengers was provided more or less directly over the lower wing, with cargo/mail compartments between the pilot's cockpit and the cabin, and between the cabin and the engine firewall.

Boeing was successful in its bid, being allocated the San Francisco-Chicago route, and was equally successful in gaining approval for its new aircraft. Following tests .for certification, the Model 40A gained Approved Type Certificate No. 2, issued by the US Department of Commerce. A total of 25 was built, 24 for service with the new Boeing Air Transport Corporation, and one as an engine test-bed for Pratt & Whitney. The first example of the Model 40A was flown initially on 20 May 1927, and all 24 for Boeing Air Transport had been delivered in time for inauguration of the company's first air mail service on 1 July 1927.