Following the usual pattern of contraction the future negative tense of “will not” should be “willn´t” but this is never used; instead we use ” won´t”. So how did we get there?
Well, “won´t” was created when an older form of “will not” was in use being formed from “woll not”, as “woll” was an alternative form of “will” up to the middle of the nineteenth century.
As the use of “woll” died out “will” became dominant but the future negative form of “woll” remained in use for “will”. Simple! Er…
For more on this, read this article, which explains in more depth the craziness that can be English! You will, won´t you?