Boeing has begun to assemble an unmanned, liquid-hydrogen powered, high-altitude long-endurance (HALE) aircraft. The twin-engine Phantom Eye will have a 150-foot wingspan and be capable of flying for more than four days at altitudes up to 65,000 feet while carrying a payload of up to 450 pounds. In addition, the Boeing Phantom Works is working on Phantom Ray, a fighter-sized unmanned aerial vehicle (UAV) that will serve as a flying test bed for advanced technologies.