On the eve of America's entry into World War II, for defense purposes, CAA extended its ATC system to include operation of airport towers. In the postwar era, ATC became a permanent federal responsibility at most airports. The postwar era also witnessed the advent of commercial jets. The British Overseas Aircraft Corporation introduced the first commercial jet service in 1952. The 36-seat Comet flew at 480 miles per hour. The top cruising speed of the DC-3 piston aircraft, in comparison, was about 180 miles per hour. By the mid-1950s, U.S. companies began designing and building their own jet airliners.