TULSA, Oklahoma - A mysterious U.S. Air Force airplane visited Tulsa Thursday afternoon.

It's a military version of the Boeing 747 called the E-4B Nightwatch. The Pentagon uses the plane for what it calls the National Airborne Operations Center or NAOC for short.

In a national emergency, the president, secretary of defense and/or the Joint Chiefs of Staff would use it to command of the military. The idea is that it's so mobile, it would be very hard for enemy forces to target it.

Because it's considered a national security asset, the Air Force won't talk about its operational activity, including its training flights.

Some people have mistaken it for Air Force One. Any Air Force plane carrying the president uses the call sign Air Force One, but the Air Force's primary presidential transport is the 747. The 747s used to carry the president are significantly different both inside and out from the 747s used as the Nightwatch. 

The easiest differences to spot are in the paint schemes. The presidential 747s have much more blue than the E-4Bs. The E-4Bs also have a distinctive hump behind the cockpit.

The crews who fly it spend most of their time training, which includes practicing takeoffs and landings. Tulsa is a perfect place to do that when the weather is clear, because the airport has relatively light traffic and a very long runway. 

The airplane has visited Tulsa in 2011, 2010 and 2008.

The E-4Bs were first deployed in 1974, when they were known as the National Emergency Airborne Command Post or NEACP, pronounced "Kneecap."

In 1994, NEACP's name was changed to NAOC and the aircraft took on another responsibility: carrying FEMA crews to the sites of natural disasters where it serves as a temporary command post on the ground until more permanent facilities can be set up.

E-4Bs are flown by the 1st Airborne Command and Control Squadron from the 55th Wing at Offutt Air Force Base in Omaha, Nebraska and the Squadron has a total of four. They were moved from Andrews Air Force Base in Maryland to the Midwest so they'd be safer from attack.

One E-4B and its crew stays on full alert at all times. One stays relatively close to Air Force One so that the President can access it quickly from anywhere in the world.