Boeing C-137 Stratoliner 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 USAF: 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 non-presidential C-137C aircraft were later added. To supplement its VC-137s, the USAF converted
Boeing VC-25 board are connected with approximately 238 miles (383 km) of wiring, twice that of a regular 747. All wiring is covered with heavy shielding for protection from a nuclear electromagnetic pulse in the event of a nuclear attack. The aircraft also has electronic countermeasures (ECMs) to jam enemy radar, flares to avoid heat-seeking missiles, and chaff to avoid radar-guided missiles. All small arms and ammunition stores not under the physical possession of the Secret Service on board the VC-25s are stowed and secured by the Secret Service in separate locked compartments, each with a different locking mechanism for added security.