What is the origin of "Keep Calm And..." ?

In My Humble Opinion
1

I see this all over the place. There’s always a little crown logo over the text.

Do a Google Image Search for examples.

Oh wait, let me link it for you.

2

There were made up for the British people in WW II as Keep Calm and Carry On.

http://www.keepcalmandcarryon.com/history/

3

It was a poster for WWII that the British government printed but then didn’t use. Cite. Way more detailed cite from the horse’s mouth.

4

Thanks!