I’m currently scanning in some of my paperwork from what was then The Public Record Office, now The National Archives, and found the start of this interesting letter from 17th July 1939 (from AP Waterfield to Ivison Macadam):
I am troubled about this Poster Question. We must get the right idea across, and so far I can’t feel that we have got it at all. The “Keep Steady”, “Keep Calm”, doesn’t, I feel sure, hit it off: it’s too commonplace to be inspiring, and it may even annoy people that we should seem to doubt the steadiness of their nerves. And worst of all, it implies that we are on the inferior side, on the defensive from the start against a superior enemy who have already seized the initiative. That surely is all wrong. What we need, I feel sure, is a rallying war-cry that will bring out the best in everyone of us and put us in an offensive mood at once.
From INF 1/226. Read more about the first posters from the Second World War.