In the early 1930s Boeing became a leader in all-metal aircraft construction, and in the design revolution that established the path for other transport aircraft through the 1930s. In 1930, the Monomail, a low-wing monoplane that carried mail, was built. Built entirely out of metal, it was very fast and aerodynamic, and had retractable landing gear. In fact, its design was so revolutionary that the engines and propellers of the time were not adequate to realize the potential of the plane. By the time controllable pitch propellers were developed, Boeing was building its Model 247 airliner. Two Monomails were built. The second one, the Model 221, had a 6-passenger cabin. In 1931, the Monomail design became the foundation of the Boeing YB-9, the first all-metal, cantilever-wing, monoplane bomber. Five examples entered service between September 1932 and March 1933. The performance of the twin-engine monoplane bomber led to reconsideration of air defense requirements, although it was soon rendered obsolete by rapidly-advancing bomber designs.
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