There is nothing quite like the U.S. Air Force’s E-4B aircraft. Born and bred for battle, the modified Boeing 747s are nearly six stories tall, equipped with four colossal engines and designed to survive a nuclear attack.

“It's like a backup Pentagon,” a U.S. Air Force crew member told CNBC aboard one of the aircraft, which was transporting Defense Secretary James Mattis at Marine Corps Air Station Miramar, California this past week.

“There’s always one plane on alert and ready to go 24 hours, seven days a week,” he added.

The E-4B Advanced Airborne Command Post - or “doomsday plane” - is a highly survivable flying fortress.

And much like Air Force One, its sister aircraft with the same recognizable paint job, most of the E-4B’s capabilities are classified.

Designed during the Cold War, each E-4B - or “Nightwatch” - is built to endure the immediate aftermath of a nuclear detonation. During peacetime, the plane is used to transport the secretary of Defense and his staff.

Currently, there are four of these unique aircraft in the Air Force’s fleet. The identical planes, which are based at Offutt Air Force Base in Nebraska, have been in operation since 1980.

The noticeable hump on top of the plane is called a “ray dome” and houses some of the nearly 67 different satellite dishes and antennas. The dome is one element in the E-4B’s communication portfolio that gives those aboard the ability to contact ships, submarines, aircraft and landlines anywhere in the world.

What’s more, thanks to aerial refueling and the E-4B’s massive fuel tanks, the plane can stay aloft for several days. The three-deck aircraft can support a crew of up to 112 people. It has 18 bunks, six bathrooms, a galley, briefing room, conference room, battle staff work area and executive quarters.