Submarine aircraft carriers were developed by the Imperial Japanese Navy to a greater extent than any other navy, before and during World War II. In total, 42 were built, as listed below (other sources say 47). Although other navies had experimented with submarine aircraft carriers, by World War II the IJN was the only navy (aside from one fielded by the French Navy) using them. They had little effect on the war, although two were used to carry out attacks on the continental United States.