The revolutionary Boeing Model 247, developed in 1933, was an all-metal, twin-engine airplane and the first modern passenger airliner. Model 247 had a gyro panel for instrument flying, an autopilot, pneumatically operated de-icing equipment, a variable-pitch propeller and retractable landing gear. Seventy-five 247s were built. Boeing Air Transport flew 60 Model 247s. United Aircraft Corp. flew 10, and the rest went to Deutsche Lufthansa and a private owner in China. With its powerful engines and its single cantilevered wing, the 247 gave United Air Lines the ability to offer 10 round trips daily between New York and Chicago. The 247s remained in airline service until World War II, when several were converted into C-73 transports and trainers. Some were still flying in the late 1960s.