Wonderful post on AviationIntel.com about the history of the US’ newest spyplane, the RQ-170 Sentinel which was used in the Osama Bin Laden raid and most famously was forced to land in Iran.  The post traces the origin of the airplane from early technology demonstrators.  I love reading about the covert programs of the US military and intelligence programs and wonder what other yet-to-be-disclosed airplane programs there are out there.
http://aviationintel.com/2011/12/28/rq-170-sentinel-origins-darkstar-has-grown-up/