The 216th Army Air Forces Base Unit (Special) constructed prototype atomic weapons (without nuclear material) and drop tested them. Little was known about the flight characteristics of the prototype atom bomb designs and how the fusing mechanism would work. In February 1945, a Flight Test Section was created within the 216th Army Air Forces Base Unit to carry out testing with prototype bombs in the shape of the Little Boy and Fat Man bombs. It was originally equipped with five Silverplate B-29s, three flight crews and five maintenance crews. To help out with an increasingly demanding schedule, four crews from the 393d Bombardment Squadron were made available. The Flight Test Section carried out 24 drop tests in June and 30 in July. About two-thirds of the June tests were with Fat Man shapes and the rest with Little Boy ones. In July, all but four of the tests were with Fat Man shapes, some with explosive-filled Pumpkin bombs. Test drops were carried out at Wendover, at the Naval Ordnance Test Station at Inyokern, and at the Naval Auxiliary Air Station Salton Sea. Testing continued up to the last minute, with the Fat Man firing unit, known as the X-unit, only being successfully tested at Wendover on 4 August, and a final test of the X-unit was carried out six days later.