The Boeing E-4 Advance Airborne Command Post (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-4B evolved form the E-4A, which had been in sevice since late 1974.  The first B-model was delivered to the Air Force in January 1980.  By 1995, all aircraft were converted to E-4B standard.

In August 1994, the E-4B assumed an additional role.  The E-4B will support the Federal Emergency Management Agency's (FEMA)  request for assistance when disasters occurs.

There are four E-4B currently operated by the United States Air Force and are assigned to the 1st Air Combat Command Control Squadron (1ACCS) of the 55th Wing at United States Strategic Command.  The first mission for the aircraft was known as NEACP (National Emergency Airborne Command Post)

The E-4B is a militarized version of the Boeing 747-200.  It provides approximately triple the floor space of its predecesor, the EC-135 Looking Glass airborne command post.  The aircraft has electomagnetic pulse protection, an electrical system designed to support advanced electronics and a wide variety of new communications equipment.  Other improvements include nuclear and thermal effects shielding, acoustic control, an improved technical control facility and an upgraded air-conditioning sytem for cooling electrical components.

The E-4B still serves as the National Airborne Operations Center (NAOC) for the National Command Authorities.