Where did that expression come from?

A recent TV programme had one of the characters saying something like "when this is all over, I want to buy a little farm in Provence and settle down".  And this brought to mind that it was commonplace in the last war for fighter pilots to say much the same thing - except of course that their wish farm would be in the English countryside.  So when a pilot was killed it would be said that he had "bought the farm", which later got shortened to just "He's bought it".