1/The B-3 was developed because of the need for a winter flying jacket for air postal duty, which the army was responsible for. It began use on May 8, 1934 with the A-3 trousers, worn as a combination . It was constructed of sheepskin and reinforced with horsehide pieces. When the war started and huge needs of the B-3 occurred with it, the production cost became a problem because of the luxurious materials needed. It was in limited use by 1943 Available in DuoTone or MonoTone Version 2/The B-6 was developed as a lighter jacket, which was easier to move around in. It was developed in 1939, and used officially from June 12th. The B-6 features action pleats and zipper adjustments on the waistline to allow a better fit and make more activity possible. Because the sheepskin is thinner than in the B-3, the B-6 was worn in combination with the A-5 trousers as a lightweight winter suit. 3/Strictly speaking, the D-1 was not a flight jacket. This was a sheepskin jacket designed for mechanics and ground crew. The D-1 was a very simple jacket while providing adequate protection from the cold.