"Keep Calm and Carry On" was a conceived poster used in British war time propaganda during WWII and the what seemed to be an impending invasion by German forces. Unfortunately it wasn't actually widely distributed. 2.5 million copies were made but never made it out of storage.
An article by the BBC entitled "The greatest motivational poster ever?" explains in 1939 the British government had three posters designed with the purpose of "steadying the the public's resolve and maintaining moral." The only guidelines were that it have a bold red background and image of the crown of King George the VI. The other slogans were "Freedom is in Peril" and "Your Courage, Your Cheerfulness, Your Resolution Will Bring Us Victory".
The poster was only recently found in 2000, discovered in a box bought at auction by a man named Stuart Manley. This man was a bookseller and liked the poster so much he had it framed and displayed in his shop. After that, the slogan became a phenomena of its own.
Today, this poster is subject to as many imitations and variations as such iconic logos as Milton Glaser's "I (HEART) NY".
SOURCE: http://news.bbc.co.uk/2/hi/uk_news/magazine/7869458.stm
