Boeing 247 airliner. Passengers in the 1930s boarding a United Airlines and National Air Transport aircraft that is also being used as a US Mail flight (AM17 Express, shown on the tail of the plane). The aircraft's civil registration number (NC13334) is shown on the right-hand wing. United Airlines and National Air Transport were originally part of Boeing, but were split off from that company after the Air Mail Act of 1934 that required manufacturers and airlines to be separate companies. The Boeing 247 was introduced in 1933. It had a wingspan of 23 metres, a length of 16 metres, a crew of 3, and could carry 10 passengers.