This plane flying above you could mean you have just minutes left to live

Many would look twice if they saw a plane as big as a five-story building in the sky - and unfortunately, if you see one above you, it may be a sign of bad things to come.

The US has four E-4B Nightwatch aircraft, often nicknamed “Doomsday Planes, which are modified Boeing 747-200s.

If the United States ever finds itself in a nuclear attack, the planes are designed to keep operating despite some of the effects of the nuclear blast. Therefore, if a nuclear war destroys normal military command centres, the plane gives US leaders a place where they can communicate with and direct the military.

The role of the planes are particularly significant amid rising tensions between the US and Iran, with President Trump threatening to 'bomb the [expletive] out of' Oman if they move forward with negotiations with Iran, over a plan to reopen the Strait of Hormuz.

The E-4B is the US Air Force's militarized version of a Boeing 747 and it towers 63.5 feet above members of the US Air Combat Command Security Forces guarding it during a trip to South Korea in 2013 (Jacquelyn Martin-Pool/Getty Images)

In the words of the US Air Force, here's the E-4B, is purposed for.

Its mission statement reads: "The E-4B 'Nightwatch' serves as the National Airborne Operations Center and is a key component of the National Military Command System for the President, the Secretary of Defense and the Joint Chiefs of Staff.

"In case of national emergency or destruction of ground command and control centers, the aircraft provides a highly survivable command, control and communications center to direct U.S. forces, execute emergency war orders, and coordinate actions by civil authorities.

"The conduct of E-4B operations encompasses all phases of the threat spectrum. Additionally, the E-4B provides outside the continental United States travel support for the Secretary of Defense and his staff to ensure Title 10 command and control connectivity."

However, they can also be used for routine missions.

An air-to-air right side view of an E-4B advanced airborne national command post aircraft being refueled from a KC-135 Stratotanker aircraft (USAF)

To put it in black and white, it's in the event the White House and the Pentagon are nuked. I mean, Washington, D.C. would probably be a target for two dictators we've come to know rather well in recent years - North Korea's Kim Jong Un and Russia's Vladimir Putin.

Although there are four of these planes, each of them is stored at the Offutt Air Force Base, Nebraska, which is home to the US Strategic Command.

So what makes these planes so good that they can withhold nuclear blasts then?

Let's start with the sheer size of them. They are 231 feet long, 63 feet high and have a wingspan of 195 feet, while they weigh 800,000 pounds - that's the equivalent of flying 80 African elephants through the sky.

They've got four electric turbofan engines that each pack a punch - the thrust power being 52,000 pounds for each engine - so 208,000 pounds in total.

They are also kitted out with nuclear and thermal effects shielding and an air-conditioning system for cooling electrical components, as well as the capability to refuel in-flight and can fly for 12 hours straight before needing to.

They cost a staggering $223.2 million, and one of the four planes is always on alert 24 hours a day 'to provide direct support to the President, the Secretary of Defense, and the JCS' (Joint Chiefs of Staff).