The Big Brother, Doomsday!
If Air Force One needs a backup, its larger brother steps in. E-4B, often known as “Doomsday Plane,” is his name. Designed to protect against a nuclear assault during the Cold War, the E-4B is an airborne nuclear bunker and command center. In the event of an emergency, Doomsday is constantly ready, following Air Force One. According to reports, there are four Doomsday Planes in existence.
Source: Tumblr
Air Force One and “Doomsday” can both stay in the air for days and survive even a nuclear blast. The majority of the Doomsday planes’ capabilities are classified as those of Air Force One planes.