I'm not saying it's aliens, but...

The size is very hard to determine, for example, although the image size at contrailing height suggests that it is bigger than an X-47B. However, the basic shape - while it resembles Boeing's Blended Wing Body studies or the Swift Killer Bee/Northrop Grumman Bat unmanned air system - is different from anything known to have flown at full size, lacking the notched trailing edge of Northrop Grumman's full-size designs. 

(link)