I've just been through the proposal for the Dartmouth AI conference of 1956, and it's a surprising read. All I really knew about it was its absurd optimism, as typified by the quote:
An attempt will be made to find how to make machines use language, form abstractions and concepts, solve kinds of problems now reserved for humans, and improve themselves. We think that a significant advance can be made in one or more of these problems if a carefully selected group of scientists work on it together for a summer.
But then I read the rest of the document, and was... impressed. Go ahead and read it, and give me your thoughts. Given what was known in 1955, they were grappling with the right issues, and seemed to be making progress in the right directions and have plans and models for how to progress further. Seeing the phenomenally smart people who were behind this (McCarthy, Minsky, Rochester, Shannon), and given the impressive progress that computers had been making in what seemed very hard areas of cognition (remember that this was before we discovered Moravec's paradox)... I have to say that had I read this back in 1955, I think the rational belief would have been "AI is probably imminent". Some overconfidence, no doubt, but no good reason to expect these prominent thinkers to be so spectacularly wrong on something they were experts in.