From The Airport Wiki

The Boeing Model 40 was a mailplane that became the first aircraft built by the Boeing company to carry passengers. It was of conventional biplane configuration with a combination of standard and warren-truss style interplane struts. Originally designed to compete for a US Mail contract in 1925, it was rejected in favour of the Douglas M-2.

The design was revived in 1927 as part of Boeing's tender for newly-privatised airmail routes. Designated the Model 40A, this variant was powered by an air-cooled Pratt & Whitney Wasp engine, which offered a 200 lb weight saving over the Liberty Engine originally specified by the postal service in 1925. Although the primary purpose of the aircraft was to carry mail, two passengers could be accommodated in the small cabin, allowing Boeing to operate it on any of the routes that the firm might bid for. The original fuselage design was changed to one using welded steel tubing.

Boeing successfully tendered for the San Francisco-Chicago route, and Boeing Air Transport Corporation commenced operations on 1 July 1927 with 24 Model 40As.