The US Air Force's fleet of four Boeing E-4Bs are national command posts intended for use in the event of nuclear war. But they have been used for various other purposes military and civilian since they haven't been needed, thankfully, for their primary mission.
A quick briefing on the U-2 Dragon Lady, RC-135 Rivet Joint, F-15 Eagle, AN/TWQ-1 Avenger variant of the Humvee and Japan's ShinMaywa SU-2 STOL/Amphibious aircraft