Officially, aliens have never existed but flying saucers very nearly did. The National Archives has recently published never-before-seen schematics and details of a 1950s military venture, called Project 1794, which aimed to build a supersonic flying saucer.
The newly declassified materials show the U.S. Air Force had a contract with a now-defunct Canadian company to build an aircraft unlike anything seen before. Project 1794 got as far as the initial rounds of product development and into prototype design. In a memo dating from 1956 the results from pre-prototype testing are summarized and reveal exactly what the developers had hoped to create.
Source: Wired. Read full article. (link)