MARINE CORPS AIR STATION MIRAMAR, Calif. — There is nothing quite like the 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. "There's always one plane on alert and ready to go 24 hours, seven days a week."

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.