... were both released in 1964, in October and January respectively.
Stanley Kubrick's Dr. Strangelove was a comedy satire, and something of a tour de force for Peter Sellers; Fail Safe, directed by Sidney Lumet, was a pure drama.
Dr. Strangelove took just under $9.5 million at the US box office; it was the 15th highest grossing film of 1964. (The top box office hit that year was My Fair Lady, which grossed $72 million.) The figures for Fail Safe appear to be lost in the mists of time; according to Wikipedia "it garnered excellent reviews, but its box office performance was poor." Not a blockbuster then; but for me, even Dr. Strangelove's figures don't justify that description.
As for Fail Safe being "a serious version" of Dr. Strangelove: the setter doesn't say who considers this to be the case, but I don't think it does justice to either film.
I'm being picky, I know, but for me the question would have been better phrased something like "Sidney Lumet's 1964 Cold War drama Fail Safe had a similar plot to which Stanley Kubrick satire, released in the same year?"
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