From Defense Industry Daily:

In December 2005, the U.S. Air Force awarded Boeing a contract as Product Support Integrator (PSI) for the USAF’s E-4 National Airborne Operations Center fleet. These four 747-200 derivatives were introduced in 1974, and serve as complete flying command posts for national and military authorities. As one might imagine, they are hardened to resist the side-effects of nuclear attack, such as electro-magnetic pulse effects.

The 2005 contract was a 5-year, indefinite delivery/indefinite quantity contract vehicle, with one 5-year option and a $2 billion cost cap. It continues a long history of support from Boeing.