In 1930, Boeing created the revolutionary Monomail, which made traditional biplane construction a design of the past. The Monomail wing was set lower, was smooth, made entirely of metal and had no struts (cantilevered construction). The retractable landing gear, the streamlined fuselage and the engine covered by an antidrag cowling added up to an advanced, extremely aerodynamic design. The 221A was a revised version designed for transcontinental passenger service.