Douglas Y1B - 7 The Douglas Y1B - 7 was a 1930s United States bomber aircraft . It was the first US monoplane given the " B - " ' bomber ' designation . The monoplane was more practical and less expensive than the biplane , and the United States Army Air Corps chose to experiment with monoplanes for this reason . At the time the XB - 7 was ordered , it was being tested by Douglas Aircraft as an observational plane . Design and development . In 1929 – 1930 , the Douglas Aircraft Company designed a new twin - engined monoplane observation aircraft to compete with the Fokker XO - 27 , two prototypes of which had been ordered by the United States Army Air Corps in July 1929 , as Douglas feared that the advanced Fokker would challenge Douglas ' s role as the major supplier of observation aircraft to the Air Corps . The Douglas proposals resulted in the Air Corps placing an order for two prototypes , the XO - 35 and XO - 36 , on 26 March 1930 . The two aircraft were to differ only in the engines fitted , with the XO - 35 having geared Curtiss V - 1570 - 29 Conqueror engines , while the XO - 36 used direct - drive V - 1570 - 23 , which both produced each . The Douglas design had gull wings mounted high on the aircraft ' s fuselage and the engines suspended in streamlined nacelles under the wings by bracing struts . A retractable tailwheel undercarriage was fitted , the mainwheels retracting into the engine nacelles . The fuselage was a semi - monocoque structure with corrugated duralumin covering , and housed the aircraft ' s crew of four , a nose gunner / observer , a pilot , whose cockpit was situated ahead of the leading edge of the wing , an upper gunner sat in a cockpit aft of the wing , and a radio operator housed within the fuselage . Armament was two . 30 in machine guns . The projected performance of the new twin - engined Douglas and Fokker observation aircraft was much greater than the Keystone biplanes ( the Keystone B - 3 , B - 4 , B - 5 and B - 6 ) that equipped the Air Corps light bomber squadrons , and both Fokker and Douglas were instructed to complete one of their prototypes as a bomber , with the XO - 36 being redesignated the XB - 7 , with provision to carry up to of bombs on racks under the fuselage added . The XO - 36 made its first flight at the Santa Monica , California factory of Douglas in spring 1931 . On 22 August 1931 , the Air Corps placed an order for a small batch of service test aircraft , consisting of seven Y1B - 7 bombers and five Y1O - 35 observation aircraft . These differed from the prototypes in being having smooth fuselage skins rather than the corrugated skins used on the prototype , while the fuselage was longer to adjust the aircraft ' s center of gravity . A revised fuel system was fitted , with more fuel carried in order to give the two - hour endurance specified by the Air Corps . More powerful Conqueror engines were fitted , with all aircraft using geared engines as used on the XO - 35 . Operational history . The XO - 35 was delivered to the Air Corps at its test center at Wright Field , Ohio on 24 October 1931 , with the XB - 7 following in July 1932 . The twelve service test aircraft were completed between November 1932 and March 1933 . The Y1B - 7s equipped the 11th and 31st Bombardment Squadron s at March Field , California , while the Y1O - 35s were issued to a number of Squadrons , including the 88th Observation Squadron at Brooks Field , Texas . One notable early use of the Y1B - 7 was during a May 1933 exercise at Fort Knox , where it was able to outrun biplane fighters like the Boeing P - 12 , showing that the Air Corps needed more modern fighters . Despite the aircraft ' s superior performance to the elderly Keystone biplanes , the Douglas monoplane never entered mass production either in the observation or bomber roles because newer , more capable aircraft , such as the Martin B - 10 , were under development while a change in Air Corps policy meant that it was no longer interested in buying more twin - engined observation aircraft . In February 1934 , US President Franklin D . Roosevelt cancelled all air mail contracts owing to a scandal relating to the award of contracts , thus initiating the Airmail Emergency , where the Air Corps was tasked with carrying the mail until new contracts could be awarded . The O - 35s and B - 7s were considered some of the least unsuitable aircraft in the Air Corps inventory for the air mail role , so the six remaining Y1B - 7s and the five Y1O - 35s , together with the XO - 35 , were assigned to air mail routes in the western United States , which included hazardous operations over the Rocky Mountains , the Cascade Range and the Sierra Nevada . By the time the Air Corps involvement in air mail flights ended in June 1934 , the twelve O - 35s and B - 7s had flown 1 , 412 hours on mail flights , while four of the B - 7s had been lost as a result of accidents . The last O - 35s and B - 7s were withdrawn from use in December 1938 .