http://en.wikipedia.org/wiki/Boeing_E-4
The Boeing E-4 Advanced Airborne Command Post, with a project name of "Nightwatch", is an aircraft operated by the United States Air Force 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