Myth. Nobody thought the Earth was flat in the middle ages. Catholic doctrine did not teach Earth was flat. Europeans knew Earth was found and its correct size since 400BC. Europe didn't know about the Americas however. This, assuming the correct size of the planet, reaching Asia sailing west was impossible for fresh water and food would run out long before the ships would arrive in Japan. Columbus thought the Earth was smaller than it really is. So he thought it was possible. He got lucky. Had the Americas not existed, they would all have died 1/3 of the distance to Japan