The four-passenger-Model 40B-4 was the major production model of the first Boeing mailplane. It had extra cabin windows and could carry four people as well as 500 pounds of mail. The first Model 40 was redesigned as the Model 40A in 1927 using an air-cooled engine. This engine was about 200 pounds lighter than the water-cooled engines used by competitors for the airmail contract, so the Model 40A won and became the first Boeing-built airplane to carry paying passengers as well as mail. The Model 40 series (A, B, and C) flew under the banner of Boeing Air Transport, a new company subsidiary. Models 40X and Y were custom-built for oil companies.