The B3 introduced in 1931, not to be confused (but easily is) with the lighter weight & shorter slimmer version, the B6 or the 3/4 length B7 with coyote fur lined hood - made of horsehide with a sheepskin lining developed for bomber crews, especially the gunners crews in bays which were open to the elements in high altitudes & temperatures as low as -50 to -70c 🥶 in the B-17s &  B-24s Bomber aircraft before they were sealed in their bays with perspex blister covers in later versions of those aircraft. They had double throat straps on the huge sheepskin collar with a  peculiar design feature of only 1 pocket, there are many versions of bomber jackets B3, 6, 7, 10, 15 with the sheepskin stopping at the B10 when it was then made of cloth with an alpaca fur collar & lining, not dissimilar to the N1 Deck jacket in design & general look, while the A model jackets A1 & A2  were leather lined with wool, cotton, or silk.