Then-US President George W. Bush was heavily criticised for his inaction after hearing about the 2001 terrorist attacks.

AMERICA has a doomsday plane. Actually it has four of them. But the US government does not — repeat, DOES NOT — talk openly about its doomsday planes.

The doomsday plane is like a mirror to Air Force One — a great big flying working office for the president and his or her staff.

But while Air Force One is your everyday, garden variety kind of presidential flying office, the doomsday plane is built and equipped to keep the US government running in the event of a nuclear war or other disaster.

Here’s some of the stuff we know about it:

It’s a converted 747 called an E-4B, also referred to as the “National Airborne Operations Center” when it’s called into action, which reportedly last happened on September 11, 2001.

The doomsday plane can reportedly withstand a nuclear bomb, asteroid blasts and terror attacks while staying airborne for days without refuelling.

In fact, the doomsday plane can remain aloft for a week with mid air-refuelling. The only reason it can’t fly longer is because the engine can only be lubricated on the ground.

The doomsday plane has a radiation-protected shell, 67 antennae and satellite dishes, plus a bunch of other hi-tech gear. The communications equipment is housed in the distinctive bubble behind the cockpit.

Yet for all its technical whizzbangery, the plane was once full of clunky 80s technology like the old school computers you see in the lairs of James Bond villains a few decades ago. Much of the old technology has reportedly been pulled out now, but the cockpit is still hardly what you’d call state of the art.

Interestingly, some see that as an advantage in a national emergency. As one officer told the Discovery Channel: “You rely on more parts, it’s more susceptible to breaking”.

The doomsday plane has three decks and a crew of up to 112 people, making it easily the largest crew in any military plane in the US, and probably the world.

The doomsday plane was commissioned by Ronald Reagan in the 1980s in the dying days of the Cold War when tensions were exceptionally high and the fear of nuclear war felt genuine and frightening.

@AriFleischer I logged a few miles on the E4 "doomsday" on SECDEF trips to the Gulf in the 90's. Not as comfortable as AF1 though. — Sean Agnew (@seanagnew) September 11, 2014

US authorities rarely discuss the doomsday plane. In particular, they don’t mention the fact that its distinctive form was spotted by reliable witnesses above the heavily restricted air space above the White House on the morning of September 11, 2001, around the time the twin towers were struck by hijacked planes, but before the Pentagon was hit.

Conspiracists have long seen this as evidence that the doomsday plane was in effect a command centre for the 9/11 attacks, which they consider an inside job.

There is no mention of the doomsday plane’s deployment that day in the official report of the 9/11 Commission, an omission which has fuelled such conspiracies.

In all likelihood, the plane was never mentioned because the US government doesn’t exactly advertise the doomsday plane’s existence or its activities. That’s not a particularly sexy theory but it’s probably pretty close to the truth.

Whatever the case, here’s a tweet from former George Bush press secretary Ari Fleischer, who caught a glimpse of it back on that fateful day.