The Monomail of 1930 was developed by Boeing as one of the first truly "modern" transport airplanes, the Monomail led the way to general acceptance of the all-metal low-wing monoplane with retractable landing gear as the standard airline type. Known as the Boeing Model 200, the first Monomail was an all-cargo airplane, with open pilot's cockpit to the rear. Boeing Monomail 221 was generally similar but carried eight passengers in a forward compartment. Both models were powered by 575 hp Pratt & Whitney Hornet engines. The Model 200 and the 221 were later modified into Model 221-As and were combination passenger-cargo airplanes. The Monomail led to the Boeing B-9 bombers and later the highly successful Boeing 247 Transport.