Discovered by the Allied forces in the closing months of WW2, this German engineered stealth jet fighter aircraft had been well into development.
A team of modern aircraft engineers from Northrop Grumman constructed a full scale replica of the aircraft with 1940’s materials in order to test the stealth technology of the time.
After an expenditure of about $250,000 and 2,500 man-hours Northrop’s Ho-229 replica was tested at the company’s classified radar cross-section (RCS) test range at Tejon, California, where it was placed on a 50-foot articulating pole and exposed to electromagnetic energy sources from various angles, duplicating the same three frequences used by the Chain Home radar network of the British in the early 1940s. RCS testing showed that an Ho-229 approaching the English Coast from France flying at 550 mph at 50 to 100 feet above the water would not have been visible to Chain Home radar, while a Bf 109 or Fw 190 was visible up to 80 miles away
link to National Geographic article