Inside the 'Doomsday plane' that follows Donald Trump around in case nuclear war breaks out

Whenever the US President's Air Force One is airborne, this 'Doomsday plane' is never far behind.

Rare footage captured on board one of four E-4B Nightwatch planes provides a glimpse into the aircraft tasked with following Donald Trump when he flies across the world.

The planes act as the 'flying Pentagon' in the case of a nuclear or terrorist attack, or if war is declared on the United States.

They are all modified Boeing 747s kitted out with specialist equipment and crammed full of military strategists, analysts and communication aides.

Known as the National Airborne Operations Center, the E-4Bs follow the better-known Air Force One in case nuclear war breaks out.

They act as a command centre for the President, US Defense Secretary and Joint Chiefs of Staff and follow Air Force One whenever it takes to the skies.

The planes have been in operation since the 1970s, during the Cold War, and can make contact with anyone in the world at any time.

They are designed to withstand an electromagnetic pulse sparked by a nuclear explosion and contain top secret defence measures to protect themselves from an attack.

While they also includes a five-mile trailing antenna to allow the President to remain in contact with a nuclear submarine fleet even after ground communication had been destroyed.

They are deemed so crucial in the event of a national catastrophe, they have been dubbed by crews as "Air Force One when it counts", Politico reported.

When the President is in the US, one E-4B will have an engine running at its base in Nebraska 24 hours a day so it is ready to launch in an emergency and meet the President almost immediately.

While the planes are not kept secret, they are barely mentioned to the point the Air Force does not publicly acknowledge owning some of them.