The Boeing Monomail, first flown in 1930, was ahead of its time. The low-wing cargo and mail plane did not have struts under the wing, and had smooth, all-metal construction with anti-drag cowling over an air-cooled engine. However, the design did not reach its full potential because it needed a variable-pitch propeller. The Monomail did not acquire this type of propeller until the newer, multi-engine airplanes that would replace it were already flying.