The Boeing E-4 Advanced Airborne Command Post, with a project name of "Nightwatch", is an aircraft operated by the United States Air Force (USAF) and is specially built to serve as a survivable mobile command post for the National Command Authority, including the President of the United States, the Secretary of Defense, and successors. The E-4s are operated by the 1st Airborne Command and Control Squadron in the 55th Wing out of Offutt Air Force Base, Nebraska.
Two of the original 747-200 airframes were originally planned to be commercial airliners. When the airline did not complete the order, Boeing offered the airframes to the USAF as part of a package leading to a replacement aircraft for the then used NEACP EC-135J model. Under the 481B NEACP (National Emergency Airborne Command Post) program the Air Force Electronic Systems Division awarded Boeing a contract in February 1973 for two unequipped aircraft, E-4A and powered by four JT9D engines, to which a third aircraft was added in July 1973. The first E-4A was completed at the Boeing plant outside Seattle, Washington in 1973. E-Systems won the contract to install interim equipment in these three aircraft, and the first completed E-4A was delivered to Andrews AFB, Maryland in December 1974. The next two were delivered in 1975, the third differed by being powered by the GE F103 engine, which was later made standard and retrofitted to the previous two aircraft. The "A" model effectively housed the same equipment as the EC-135, but offered more space and an ability to remain aloft longer than an EC-135.
Additionally the E-4 was capable of operating the "Looking Glass" mission of the Strategic Air Command (SAC). The E-4A aircraft were capable of remotely launching retaliatory strikes from SAC missile fields. SAC subsequently relinquished the aircraft to full time use by the Office of the Joint Chiefs of Staff.
In December 1973 a fourth aircraft was contracted for, which was fitted with more advanced equipment, resulting in the designation E-4B. On 21 December 1979 Boeing delivered the first E-4B (AF Serial Number 75-0125), which was distinguished from the earlier version by the presence of a large "hump" on the dorsal surface directly behind the upper deck. This contains the aircraft's SHF satellite antenna. By January 1985 all three E-4As had been retrofitted to E-4B models. The E-4B offered a vast increase in communications capability over the previous model and was considered to be 'hardened' against the effects of electromagnetic pulse (EMP) from a nuclear blast. Hardening the aircraft meant that all equipment and wiring on board was shielded from EMP. Additional steps were taken to block radiation from the aircraft's cabin air management system and cockpit, including the novel use of the same screens used to cover the windows of microwave ovens placed over the cockpit windows.
Estimates at the time of the production of the first E-4B placed the developmental cost at nearly US$ 1 billion. The roll out cost of the fleet was placed at approximately US$ 250 million each. The E-4B is capable of operating with a crew of 48 to 112 people, the largest crew of any aircraft in US Air Force history.
In 2005 the Air Force Appropriations Committee awarded Boeing a US$ 2 billion contract for the "continued upgrade" of the E-4B fleet, a considerable amount of money considering that there are only four aircraft. This contracted work is expected to take five years to complete.