Longtime Washington, D.C., area residents are quick to advise newcomers to build in an extra 30 minutes for even the shortest of car trips. But that tip fails to account for a mysterious aircraft that some drivers mistook for a UFO.

That is what some capital city area drivers encountered late Wednesday evening. But it was no alien spacecraft despite what you might have seen on Twitter.

The social media site was busy as drivers and onlookers posted pictures and videos from I-270 in Maryland of what a Navy spokeswoman tells U.S. News & World Report was an X-47B unmanned aircraft. The Northrop Grumman-made drone was completing the final leg of a coast-to-coast journey on America's highways from Edwards Air Force Base in California to Naval Air Station Patuxent River in Maryland.

The X-47B is known in military circles as the "UCAS," short for Unmanned Combat Air System Carrier-Demonstration. A Navy fact sheet calls it "fighter-sized," while a Northrop Grumman fact sheet states the drone has a wingspan of 62 feet and a length of 38 feet. It weighs 4,500 pounds. Photos released by the Navy show the X-47B's wings hanging far over the side of the red trailer—much wider even than the cab of the big rig.

The name hints at why the Navy so covets the drone: It is being developed to take off and land from the Navy's aircraft carriers. Large drone air frames currently in the military's fleets operate from ground airstrips.

The Navy is still testing the aircraft, and plans to keep doing so for several more years before deciding whether to place the drone on aircraft carriers.

The X-47B that was spotted Wednesday night arrived at the Maryland base "a little after midnight," Paula Paige, the Navy spokeswoman says.

Navy officials considered several other options—including transporting the aircraft by ship—before "deciding this was probably the best way," Paige says, adding "at that time there is less traffic and fewer distractions."

Paige was unable to say whether bringing naval aircraft and other equipment into the Patuxent River base via civilian roads is standard practice. A base press release did, however, note "it was not an ordinary morning...when a large flatbed truck hauling the fighter-sized X-47B Unmanned Combat Air System Demonstrator entered through the naval air station's gate."

John T. Bennett covers national security and foreign policy for U.S. News & World Report. You can contact him at jbennett@usnews.com or follow him on Twitter.