500
According to Wikipedia there exist 12 different variants of the KC-135. Name and describe them all.
KC-135A
Original production version powered by four Pratt & Whitney J57s, 732 built. Given the Boeing model numbers 717-100A, 717-146 and 717-148.[39]
NKC-135A
Test-configured KC-135A.
KC-135B
Airborne command post version with 17 built equipped with turbofan engines. Provided with in-flight refueling capability and redesignated EC-135C.[40] Given the model number 717-166.[39]
KC-135D
All four RC-135As (Pacer Swan) were modified to partial KC-135A configuration in 1979.[41][42] The four aircraft (serial numbers 63-8058, 63-8059, 63-8060 and 63-8061) were given a unique designation KC-135D as they differed from the KC-135A in that they were built with a flight engineer's position on the flight deck.[43] The flight engineer's position was removed when the aircraft were modified to KC-135 standards but they retained their electrically powered wing flap secondary (emergency) drive mechanism and second air conditioning pack which had been used to cool the RC-135As on-board photo-mapping systems.[44] Later re-engined with Pratt & Whitney TF33 engines and a cockpit update to KC-135E standards in 1990 and are were retired to the 309th AMARG at Davis-Monthan AFB, AZ in 2007.[42][45]
KC-135E
Air National Guard and Air Force Reserve KC-135As re-engined with Pratt & Whitney TF-33-PW-102 engines from retired 707 airliners (161 modified). All E model aircraft were retired to the 309th AMARG at Davis-Monthan AFB by September 2009 and replaced with R models.[9][46]
NKC-135E
Test-configured KC-135E.
KC-135Q
KC-135As modified to carry JP-7 fuel necessary for the SR-71 Blackbird, 56 modified,[40] survivors to KC-135T.
KC-135R (1960s)
4 JC/KC-135As converted to Rivet Stand (Later Rivet Quick) configuration for reconnaissance and evaluation of above ground nuclear test (55-3121, 59-1465, 59-1514, 58-0126, 58-0126 replaced 59-1465 after 59-1465 crashed in 1965). These aircraft were powered by Pratt & Whitney J57 Engines and were based at Offutt AFB, Nebraska.
KC-135R
KC-135As and some KC-135Es re-engined with CFM-56 engines, at least 361 converted.
KC-135R(RT)
Receiver-capable KC-135R Stratotanker, 8 modified with either a Boeing or LTV receiver system and a secure voice SATCOM radio.
KC-135T
KC-135Q re-engined with CFM-56 engines, 54 modified.
EC-135Y
An airborne command post modified in 1984 to support CINCCENT. Aircraft 55-3125 was the only EC-135Y. Unlike its sister EC-135N, it was a true tanker that could also receive in-flight refueling. Pratt & Whitney TF-33-PW-102. Currently retired to 309th AMARG at Davis-Monthan AFB, AZ.