The Martin B-10 was the first all-metal monoplane bomber to be regularly used by the United States Army Air Corps, having entered service in June 1934.[1] It was also the first mass-produced bomber whose performance was superior to that of the Army's pursuit aircraft of the time. The B-10 served as the airframe for the B-12, B-13, B-14, A-15 and O-45 designations using Pratt & Whitney engines instead of Wright Cyclones. A total of 348 of all versions were built. The largest users were the US, with 166, and the Netherlands, with 121.