Boeing YB - 9 The Boeing YB - 9 was the first all - metal monoplane bomber aircraft designed for the United States Army Air Corps . The YB - 9 was an enlarged alteration of Boeing ' s Model 200 Commercial Transport . Design and development . In May 1930 , Boeing had flown its Model 200 Monomail single - engined mailplane . The Monomail was of radical design for the time , being a semi - monocoque , stressed skin cantilever monoplane with a retractable undercarriage . The United States Army Air Corps bomber squadrons were largely equipped with slow biplanes such as the Keystone B - 6 , and Boeing decided to design and build a twin - engined bomber using the same techniques used in the Monomail to re - equip the Air Corps . Using its own money , Boeing decided to build two prototypes of its new bomber design as a private venture . The two aircraft differed only in the engines used , with the Model 214 to be powered by two liquid - cooled Curtiss V - 1570 - 29 Conqueror engines while the Model 215 had two Pratt & Whitney R - 1860 Hornet radial engines . Both aircraft were low winged cantilever monoplanes with a slim , oval cross - section fuselage accommodating a crew of five . The pilot and co - pilot sat in separate open cockpits , with the co - pilot , who doubled as the bombardier sitting forward of the pilot . Two gunners , each armed with a single machine gun sat in nose and dorsal positions , while a radio operator sat inside the fuselage . Like the Monomail , a retractable tailwheel undercarriage was used . The first of the two prototypes to fly was the radial powered Model 215 which , carrying civil markings and the aircraft registration " X - 10633 " , made its maiden flight on 13 April 1931 . It was leased to the Air Corps for testing under the designation XB - 901 , demonstrating a speed of 163 mph ( 262 km / h ) . Testing was successful , and both the XB - 901 and the as - yet incomplete Model 214 were purchased as the YB - 9 and Y1B - 9 respectively on 13 August 1931 , with an order for a further five for service testing following shortly . The Y1B - 9 ( " Y1 " indicating funding outside normal fiscal year procurement ) , powered by two liquid - cooled Curtiss V - 1570 - 29 ' Conqueror ' engines , first flew on 5 November 1931 . The increased power from these engines , combined with increased streamlining of the engine nacelles , increased its top speed to 173 mph ( 278 km / h ) . The YB - 9 , meanwhile , had been re - engined with more powerful Hornets , demonstrating slightly better performance than the Y1B - 9 , which was therefore also re - engined with Hornets . With the exception of the B - 2 Condor , liquid - cooled engines were never used on production bombers for the United States military . The air - cooled radial engine was lighter and more reliable than the liquid - cooled engine , and less vulnerable to enemy damage . The five Y1B - 9A service test aircraft had the Pratt & Whitney R - 1860 - 11 Hornet engines which powered the re - engined YB - 9 and Y1B - 9 and a redesigned vertical stabilizer . While enclosed canopies were considered and designed , the B - 9 was never fitted with them . Although it equaled the speed of all existing American fighter aircraft , no further aircraft were built , as the Glenn L . Martin Company had flown a prototype of a more advanced bomber , the XB - 907 , which was ordered into production as the Martin B - 10 . Operational history . The first of the five Y1B - 9As entered service with the 20th and 49th Bombardment Squadrons , 2nd Bomb Group on 14 September 1932 , with all being in service by the end of March 1933 . The new bomber proved impossible to intercept during air exercises in May 1932 , strengthening calls for improved air defense warning systems . Two B - 9s were destroyed during crashes in 1933 , one of the accidents being fatal , while the remaining aircraft were gradually phased out over the next two years , with the last being withdrawn on 26 April 1935 .