Complex sentence: Most large Allied World War II aircraft bombs had very thin skins to maximize the weight of explosive a bomber could carry--this was an improvement on the early part of the war, when the explosive content of British bomb designs was low.
Split into simpler sentences: Most large Allied World War II aircraft bombs had very thin skins to maximize the weight of explosive that a bomber could carry. This was an improvement on the early part of the war, when the explosive content of British bomb designs was low.