C-121A
Initial variant, based on the civil L-749 Constellation.
Nine built.

VC-121A
Six C-121A transports converted to VIP use.
Originally designated PC-121A.
VC-121B
Similar to the VC-121A, but with the cargo door replaced by a smaller passenger door.
One built.

C-121C
Initial variant based on the L-1049 Super Constellation.
33 built.

VC-121C
VIP conversion of four C-121C aircraft.

VC-121E
Ordered by the United States Navy as a R7V-1 but modified before delivery as a presidential transport for the United States Air Force.

YC-121F
Two former United States Navy R7V-2s with Pratt & Whitney T34 turboprop engines transferred to the United States Air Force.
Designated L-1249A by Lockheed.

C-121G
Redesignation of 32 R7V-1 transports transferred from the USN to the Air Force.

TC-121G
Three C-121Gs converted to AWACS crew trainers.

VC-121G
One C-121G converted to a VIP transport.
C-121A
Initial variant, based on the civil L-749 Constellation. Nine built.
VC-121A
Six C-121A transports converted to VIP use. Originally designated PC-121A.
VC-121B
Similar to the VC-121A, but with the cargo door replaced by a smaller passenger door. One built.
C-121C
Initial variant based on the L-1049 Super Constellation. 33 built.
VC-121C
VIP conversion of four C-121C aircraft.
VC-121E
Ordered by the United States Navy as a R7V-1 but modified before delivery as a presidential transport for the United States Air Force.
YC-121F
Two former United States Navy R7V-2s with Pratt & Whitney T34 turboprop engines transferred to the United States Air Force. Designated L-1249A by Lockheed.
C-121G
Redesignation of 32 R7V-1 transports transferred from the USN to the Air Force.
TC-121G
Three C-121Gs converted to AWACS crew trainers.
VC-121G
One C-121G converted to a VIP transport.
Lockheed C-121 Constellation