Variants began with the original KC-97A transport which numbered three examples modified to an aerial tanker standard. They lost their rear loading ramps in the conversion process but received a refueling boom arm for testing the validity of the overall aircraft design. Once the proving phase was complete, the trio were converted back into their original transport guises.
The KC-97E were true tanker forms and sixty were produced, some ultimately reconverted to transport roles under the "C-97E" designation. The KC-97F introduced 4 x R-4360-59B engines of 3,800 horsepower each and were produced to the tune of 159 examples. Again, some of this lot were reconverted to transport forms, these designated as "C-97F".
The KC-97G was the definitive mark with 592 built in all. They were purpose-designed "combination" airframes able to take on the aerial tanker role or serve in the cargo transport role as needed. Underwing fuel tanks were fitted to these aircraft.
The EC-97G was an ELectronics INTelligence (ELINT) conversion applied to three KC-97G models and outfitted with specialized equipment. The C-97G were transport conversions numbering 135 aircraft from the KC-97G stock. The GKC-97G became five KC-97G models set aside to serve as ground instruction classrooms. The JKC-97G was a "one-off" model used to test General Electric J47-GE-23 turbojet engines in underwing pods - this work eventually led to the aforementioned KC-97L of which 81 were produced from the existing KC-97G stock.
The HC-97G, another G-model offshoot, designated twenty-two aircraft converted to the Search And Rescue (SAR) role. The KC-97H was another one-off used to test a specific hose-and-drogue fuel delivery system. The YC-97J designated a pair of KC-97G models outfitted with experimental 4 x Pratt & Whitney YT34-P-5 turboprop engines but the design was not furthered. The C-97K became twenty-seven KC-97G aircraft reworked as dedicated troop transports.
A sole KC-97 example was modified for super-heavy-lift duties (as the "Super Guppy") in support of the famous American Apollo (Saturn) space program. The example eventually fell under NASA ownership in 1993 and remains in flyable condition today (2018) - one of just two such aircraft of the series to claim that status.
Despite its service role as an aerial "tanker", the KC-97 was not given the "Stratotanker" name but retained the original C-97's "Stratofreighter" name by both the United States Air Force and Boeing.
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