The DC-8 was the first Douglas jet-powered transport. It entered service simultaneously with United Airlines and Delta Air Lines on Sept. 18, 1959. Powered by four jet turbine engines, the DC-8 was capable of speeds of more than 600 mph. In a test dive, it became the first commercial transport of any kind to break the sound barrier. Throughout its 14-year-long production run, the DC-8 went through seven major variants, for a total of 556 aircraft.