What is the fine-grained topic of the following text: The country’s first fixed-wing commercial airline had been in operation since 1914, featuring a ‘‘wood and muslin biplane flying boat’’ — but by the end of the war four years later, the U.S. had a surplus of pilots, hailing the industry through everything from stunt flying to deliveries of bags of mail. Only return the topic.
History of early commercial aviation in the U.S.