F-117 also comes to mind. Amazingly the first prototypes were complete in 1977, There were rumors about it but details about the plane and program were pretty

F-117 also comes to mind. Amazingly the first prototypes were complete in 1977, There were rumors about it but details about the plane and program were pretty well guarded for another decade. https://en.wikipedia.org/wiki/Lockheed_F-117_Nighthawk#Background_and_Have_Blue https://en.wikipedia.org/wiki/Lockheed_F-117_Nighthawk#Backg...