The Martin-produced Baltimore was a light-to-medium class bomber whose design was based highly on the Martin "Maryland" A-22 aircraft. The twin-engine, four-crew system was fielded en masse by forces of the United Kingdom through the Lend-Lease act (among others in limited production numbers) and would see action from 1942 through the middle of 1945 in Africa, the Middle East and Europe. By the end of the war, some 1,500 examples would be in circulation with Britain fielding the system in no fewer than five distinct marks. Italy Co-Belligerent Air Force used Martin Baltimores at bombing missions over the Balkans.