Wikipedia says "The flight jacket, or bomber jacket is a garment originally
created for pilots, which eventually became part of popular culture and apparel. In the First World War most
airplanes did not have an enclosed cockpit, which necessitated a garment that could keep pilots sufficiently warm.
While serving in France and Belgium, the Royal Flting Corps pilots had already begun wearing long leather
coats in 1915, and the trend caught on. The US Army established the Aviation Clothing Board in September 1917
and began distributing heavy-duty leather flight jackets; with high wraparound collars, zipper closures with wind
flaps, snug cuffs and waists, and some fringed and lined with fur. Thus, the American flight jacket was born."
For more infomation check out bomber jacket history.