Matthew Zuckerman writes in his fantastic article If There's An Original Thought Out There, I Could Use It Right Now: The Folk Roots of Bob Dylan:
Original song: No More Auction Blues (trad)
Dylan song: Blowin' in the Wind (April 1962)
The tune of "Blowin' in the Wind" was (according to Dylan) based loosely on the traditional "No More Auction Blues," found on The Bootleg Series 1-3.* The guitar part is certainly very similar, though I had listened to both songs many times without noticing the resemblance .... Dylan probably learned this one from Odetta, who sang it on her live Carnegie Hall album which was recorded on April 8, 1960.
* The Bootleg Series, vol. 1-3 is an amazing collection of previously unreleased recordings from 1961 to 1989, including many of Dylan's greatest songs that would otherwise have been left in the vaults. While there are a number of compilations on the market, this is probably the finest single collection.