The twin-engine B-18 Bolo was the first Douglas medium bomber. It was a combat version of the DC-2 commercial transport, absorbed punishment well and was especially useful during the early days of World War II. Douglas produced 370 of the production model B-18 Bolos, and their availability during the late 1930s allowed the Air Corps to train bomber crews. The B-18 Bolos made up most of the bombers deployed outside the country as the United States entered World War II.