The Boeing C-137 Stratoliner is a retired VIP transport aircraft derived from the Boeing 707jet airliner used by the United States Air Force. Other nations also bought both new and used 707s for military service, primarily as VIP or tanker transports. In addition, the 707 served as the basis for several specialized versions, such as the E-3 Sentry AWACS aircraft. The designation C-18 covers several later variants based on the 707-320B/C series. The C-137 should not be confused with the similar Boeing C-135 Stratolifter; although they share a common ancestor the two aircraft have different fuselages, among other structural differences.
US Air Force procurement of the Boeing 707 was very limited, amounting to three Model 707-153s designated VC-137A. When delivered in 1959 these had four 13,500 lb (6123 kg) dry thrust Pratt & Whitney J57 (JT3C6) turbojets; when subsequently re-engined with 18,000 lbf (80.1 kN) dry thrust TF33-P-5 (JT3D) turbofans they were redesignated VC-137B. Only one other variant served with the Air Force: this was the VC-137C Air Force One Presidential transport, the two examples of which were Model 707-320B Intercontinentals with specialized interior furnishings and advanced communications equipment. Two further C-137C aircraft lacking Presidential transport modifications were later added.
To supplement its VC-137s, the USAF converted several C-135 airframes to VC-135 VIP standard, and these were used for staff transport mainly within the United States.
The C-18 is the US military designation for the conversions of the 707-320B series.
Eight second-hand (former American Airlines) 707-323Cs bought as crew trainers for the EC-18Bs, four later converted to EC-18B, two converted to EC-18D, one to C-18B; one was not taken into service and was used for spares.
Four C-18As modified alongside examples of the C-135 for Advanced Range Instrumentation Aircraft (ARIA) missions in support of the Apollo space program. The designation E-7 was originally applied to modified Boeing 707s before being replaced by the EC-18 designation.
Original designation for two prototype J-STAR aircraft, later redesignated E-8A.
Two C-18As modified as a Cruise Missile Mission Control Aircraft (CMMCA).
The two VC-137Cs were redesignated when downgraded from presidential use. SAM 26000 and SAM 27000 were retired in 1998 and 2001 respectively. Both are now in aviation museums. Two further C-137Cs were acquired by the USAF on 24 March 1988, one 707-396C (a seized aircraft formerly used for arms smuggling acquired in 1985) and one 707-382B bought second hand in 1987. Their assigned tail numbers were 85-6973 and 85–6974. Tail 85-6973 would later be converted into an E-8C JSTARS. 
Two aircraft built as Early Warning and Control System prototypes. Later re-engined and re-designated E-3A. A further second-hand 707-355C aircraft was acquired and configured as an airborne special operations command post.
A version of the 707-320, it operates as an airborne command post and communications center, relaying instructions from the National Command Authority. Its role in relaying to the fleet ballistic missile submarines, known as "Take Charge and Move Out", gives it the suffix TACAMO. Only one version of the E-6 currently exists, the E-6B. The E-6B is an upgraded version of the E-6A that now includes a battlestaff area for the USSTRATCOM Airborne Command Post
The E-8C Joint Surveillance Target Attack Radar System (Joint STARS) is a USAF airborne battle management and command and control (C2) platform that conducts ground surveillance to develop an understanding of the enemy situation and to support attack operations and targeting that contributes to the delay, disruption and destruction of enemy forces.
The Israeli Air Force operates an undisclosed number (sources suggest 7) of converted Boeing 707s with flying booms in 120 ("Desert Giants") Squadron. Israel's fleet are former civilian aircraft adapted for military uses such as aerial refueling of fighter jets and transport. Able to carry 20 extra fuel tanks while modified for aerial refueling, the Re’ems can be adapted to carry passengers as well as cargo such as military equipment and ammunition. Following the outbreak of the COVID-19 pandemic in 2020, the planes have also been used to carry medical equipment.
The 707 Tanker/Transport. Italy purchased and converted four 707s, two to tankers and two to a straight freighter. No 707 tankers remain operational as of 3 April 2008. Also, Omega Aerial Refueling Services operates K707 tankers for lease.
Three ex-Sabena airliners converted to TCA (Trainer Cargo Aircraft) in 1989 to support the NATO NAEWF E-3A training and air transport/cargo based on Boeing 707-320B. The aircraft were capable of making dry hookups with the USAF Flying Boom air-to-air refuelling system for training of pilots that were new to NAEWF, but also served as cargo/passenger transport. The two oldest/highest time 707s were replaced by two former Luftwaffe 707s in 1999. NAEWF withdrew the TCAs in 2011.