During World War II Germany's technology was beyond that of any other country.  One of the many things they were working on was the nuclear bomb.  Had it not been for smuggled information and a few select people the German SS would have been the first to use a nuclear weapon.  World War II Germany advanced technology stunned the world and a lot of that technology is revealed in a book titled "The SS Brotherhood of the Bell" by Joseph P. Farrell.  German technology included possible "flying saucers" and new energy propulsion. 

In 1947 Kenneth Arnold reported seeing nine flying objects that looked like half saucers.  His description led to unknown flying objects becoming known as flying saucers. This link shows a photo of of how he described the object.  It is important to note that World War II came to a close in 1945.  At some point of time it was disclosed that the Nazis had a jet aircraft called the Horton Ho 229.  It is highly likely that the U.S. was in possession of several and this sighting was in fact them.

Most people know what a B-2 and a  F-117 look like but before their existence was made public they were among many objects reported as UFOs.

From 1978 through 1979 there was a TV series called Project U.F.O. by Jack Webb ("...just the facts please...") based on the U.S. Air Force files from Project Blue Book .  The TV program would present various cases investigated by the Air Force and reach a credible explanation...until the last couple of minutes when new evidence was introduced.  The last few years of Blue Book investigations were made by the Air Force Office of Special Investigations, FBI and other intelligence agencies personnel in suits reponsible for coining the words "The Men in Black".