Boeing 307 Stratoliners were the world's first fully pressurized commercial airliners. They were able to fly at 20,000 feet, which was deemed "above the weather" at that time. The Stratoliner was the first airplane to have a flight engineer as a member of the crew. The engineer was responsible for maintaining power settings, pressurization and other subsystems, leaving the pilot free to concentrate on other aspects of flying the aircraft. Boeing built just 10 of them.