It's a beautiful machine from this angle, with its markings and all the surface detail. If you told me this was a new Imperial fighter for an incoming Star Wars sequel, I would believe you. I'm not surprised people mistook it for a UFO being smuggled across America.
X-47B is a technology demonstrator built by Northrop Grumman for the US Navy, a stealth unmanned combat aerial vehicle with a 62.1-foot (18.92 meter) wingspan. It's capable of operating from an aircraft carrier, using its two bays to deliver 4,500 pounds (2,000 kilograms) of weapons to any target as far as 2,416 miles (3,889 kilometers) away.
Compare it to the current US Navy's workhorse, the McDonnell Douglas F/A-18 Hornet: it has a 40-foot (12.3 meter) wingspan, and it's capable of carrying two air-to-air AIM-9 Sidewinder missiles as far as 1,250 miles (2,000 kilometers). It can carry more weapons than X-47B—13,700 pounds, 6,214 kilograms—but that would severely cut its range. 4,000 pounds of bombs plus two Sidewinders and two external fuel tanks will only give it a range of 390 nautical miles (722 kilometers).
The next version of this drone is called X-47C. When it arrives near the end of the decade it will be even larger—with a whopping 172-foot (52.4-meter) wingspan. That will make it as big as the B-2 bomber. Theoretically, the X-47C will be able to carry 10,000 pounds (4,500 kilograms) of armament. [Defense Tech]
Here's another picture from the top, taken by Northrop Grumman on January 25.