The 132-foot-long (40-metre-long) unarmed prototype, dubbed Sea Hunter, is the naval equivalent of Google's self-driving car, designed to cruise on the ocean's surface for two or three months at a time - without a crew or anyone controlling it remotely.

That kind of endurance and autonomy could make it a highly efficient submarine stalker at a fraction of the cost of the Navy's manned vessels.

"This is an inflection point," Deputy U.S. Defense Secretary Robert Work said in an interview, adding he hoped such ships might find a place in the western Pacific in as few as five years. "This is the first time we've ever had a totally robotic, trans-oceanic-capable ship."    

For Pentagon planners such as Work, the Sea Hunter fits into a strategy to incorporate unmanned drones - with increasing autonomy - into the conventional military in the air, on land and at sea.