Based on the Lockheed Super Constellation, RC-121s served from 1956 to 1965 in two "barrier" forces, one off each coast of the North American continent to complement land-based early warning radar systems.
While barrier operations off the United States ended in 1965, the Air Force continued to operate the aircraft until the late 1970s in other areas and in other roles, including gathering electronic intelligence, psyops and weather reconnaissance.
The RC-121 was 116 feet long, with a wingspan of 126 feet. Typical flight crew was six, with 11 to 25 radar operators aboard. With a range of more than 4,000 miles, these aircraft were capable of very long patrols.