Complex sentence: In 1940 Pan Am and TWA began using the Boeing 307 Stratoliner, the first pressurized airliner in service and the first with a flight engineer in the crew.
Split into simpler sentences: In 1940 Pan Am and TWA began using the Boeing 307 Stratoliner. It was the first pressurized airliner in service and the first with flight engineer as a member of the crew.