One thing that was very important to Frank as he began working on his book was to let readers know what was happening in the flying world while we was growing up. There were so many amazing changes happening. In the aircraft industry, the demand for new planes kept manufacturers busy. Among the innovative products introduced in the 1930s was the Boeing 247, the first modern airliner, which was introduced in 1933. Over in England, Frank Whittle developed the jet plane in 1937. And in 1939, Igor Sikorsky built the first helicopter for mass production. Manufacturers now faced new challenges in machining thin-walled, complex parts for these modern marvels. Source: https://www.americanmachinist.com/heating/1930s