The Boeing Monomail, designed initially to carry cargo and mail, was one of the most revolutionary airplanes in commercial aviation history. Two were built: the Model 200 mail plane and the Model 221 six-passenger transport. Both subsequently were revised for transcontinental passenger service and became Model 221As. The Monomail achieved major performance increases not by the addition of brute horsepower, but by structural and aerodynamic refinements. Air resistance was reduced by the Monomail's smooth, all-metal construction; cantilevered (that is, without struts), low-wing design; and the anti-drag cowling over the air-cooled Hornet engine. Retracting the wheels into the wing during flight further improved the mail plane's aerodynamics.