Based on the Lockheed Super Constellation, the EC-121 Warning Star was one of the first airborne early warning (AEW) aircraft which served with both the USAF and USN throughout three decades. The Super Constellation itself saw limited military service as the C-121 transport before it was decided to adapt it for numerous other roles including fleet coverage and an airborne extension of the Distant Early Warning (DEW) Line, a series of radar stations for the air defense of North America. EC-121s were fitted with a variety of radar and electronic equipment installations, usually ventral and underfuselage radomes but also a massive rotodome which would be the precursor to modern AWACS. The Warning Star was used for coordination, early warning, and communications relay duties during the Vietnam War where they directed the first successful interception in history: a pair of MiG-17s shot down in July 1965. They were eventually replaced by the E-3 Sentry in 1978.

The prototype L-1049 airliner (converted from the original C-69 Constellation) had its maiden flight on 13 October 1950 and was ordered into military service as the C-121A transport followed by the C-121C with a longer nose. Other transport versions were the USN's R7V-1 which was later known as the C-121J after 1962 while a number were sent to the USAF as the C-121G. Warning Star variants began with the PO-1W (later WV-1) prototype conversion from the basic airliner and were followed by the PO-2W (WV-2) which after 1962 were known as the EC-121K. Other USN variants inlcuded the WV-2Q (EC-121M) Elint platform, the WV-3 (WC-121N) weather recon aircraft, and the one-off NC-121K for electronic warfare. USAF variants began with the RC-121C (later turned into EC-121C and the TC-121C trainer) and EC-121D while numerous former Navy units were converted to EC-121R sensor monitoring platforms. Final variants included the EC-121H which was also converted (along with some EC-121Ds) into EC-121T AWACS.