Black project

A black project is a highly classified, top-secret military or defense project that is not publicly acknowledged by government, military personnel, or contractors. Examples of United States military aircraft developed as black projects include the Lockheed F-117 Nighthawk stealth attack aircraft and the Northrop Grumman B-2 Spirit stealth bomber, both of which were highly classified and publicly denied until they were ready to be announced to the public. In the United States, the formal term for a black project is "special access program" ("SAP"). The money that funds these projects is called the "black budget".
Examples.
United States.
Previously classified.
Speculated or classified