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 other manufacturers, and the Post Office specification had stipulated the use of a Liberty engine, plus an ability to carry 454kg of airmail.
A conventional biplane 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 began a redesign and conversion of the Model 40 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, the composite structure fuselage was replaced with 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 for 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.
Boeing built a fleet of 25 Model 40A mail planes in five months anf their enclosed cabins and heated cockpits made possible the first regular transcontinental air passenger service by Boeing Air Transport which, like Varney Air Lines and National Air Transport, later hecame part of United Air Lines’ 13,250-mile network.
One model 40A was utilised 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 airmail service on 1 July 1927.
Engine: 1 x 313kW Pratt & Whitney Wasp radial piston
Take-off weight: 2722 kg / 6001 lb
Empty weight: 1602 kg / 3532 lb
Wingspan: 13.47 m / 44 ft 2 in
Length: 10.12 m / 33 ft 2 in
Height: 3.73 m / 12 ft 3 in
Wing area: 50.82 sq.m / 547.02 sq ft
Max. speed: 206 km/h / 128 mph
Cruise speed: 169 km/h / 105 mph
Ceiling: 4420 m / 14500 ft
Range: 1046 km / 650 miles