RC-121C: 10 produced, initial USAF variant
JC-121C: 2 converted from C-121C and 1 TC-121C as avionics testbeds
TC-121C: 9 RC-121C modified before 1962 as crew trainers
EC-121D: 73 produced 1953–55 as main USAF variant and 1 converted from C-121C, originally designated RC-121D
EC-121D Quick Look: 1 testbed for QRC-248 IFF transponder interrogator
EC-121H: 42 USAF upgrades in 1962, 35x EC-121D and 7x WV-2s transferred from the Navy
EC-121J: 2 USAF EC-121D modified with upgraded electronics
EC-121M Rivet Top: 1 EC-121D testbed for Rivet Gym cryptologic linguist electronics suite, originally designated EC-121K
EC-121Q: 4 EC-121D modified with upgraded electronics for USAF Gold Digger missions
EC-121R: 30 EC-121K / EC-121P transferred to USAF in 1966–1967 and converted to Batcat sensor signal processor
EC-121S: 5 converted for Pennsylvania Air National Guard from USAF C-121 transports
EC-121T: Final USAF variant.
A total of 22 T's were converted from 15x EC-121D and 7x EC-121H.
XW2V-1: Proposed naval development with new features such as 4 Allison T56-A8 turboprop engines, L-1649A Starliner wings and Bomarc missiles for defense.
None built; was designated L-084 due to the large differences from its predecessors.
RC-121C: 10 produced, initial USAF variant
JC-121C: 2 converted from C-121C and 1 TC-121C as avionics testbeds
TC-121C: 9 RC-121C modified before 1962 as crew trainers
EC-121D: 73 produced 1953–55 as main USAF variant and 1 converted from C-121C, originally designated RC-121D
EC-121D Quick Look: 1 testbed for QRC-248 IFF transponder interrogator
EC-121H: 42 USAF upgrades in 1962, 35x EC-121D and 7x WV-2s transferred from the Navy
EC-121J: 2 USAF EC-121D modified with upgraded electronics
EC-121M Rivet Top: 1 EC-121D testbed for Rivet Gym cryptologic linguist electronics suite, originally designated EC-121K
EC-121Q: 4 EC-121D modified with upgraded electronics for USAF Gold Digger missions
EC-121R: 30 EC-121K / EC-121P transferred to USAF in 1966–1967 and converted to Batcat sensor signal processor
EC-121S: 5 converted for Pennsylvania Air National Guard from USAF C-121 transports
EC-121T: Final USAF variant. A total of 22 T's were converted from 15x EC-121D and 7x EC-121H.
XW2V-1: Proposed naval development with new features such as 4 Allison T56-A8 turboprop engines, L-1649A Starliner wings and Bomarc missiles for defense. None built; was designated L-084 due to the large differences from its predecessors.
Lockheed EC-121 Warning Star