Wordorigins.org

The meaning of this odd phrase is to travel fast and hard in a seemingly purposeless direction. The metaphor behind the phrase is obscure. Don’t confuse this phrase with hell for leather, which is a different phrase entirely—although the use of leather may come from confusion between the two. Variants of the phrase include hellbent for election, breakfast, or Georgia.

Hellbent, meaning recklessly determined, dates to 1835 in the pages of The Knickerbocker:

A large encampment of savages,..."hell-bent on carnage.”

The full phrase first appears in 1899 in a short story by Stephen Crane:

One puncher racin’ his cow-pony hell-bent-fer-election down Main Street.

(Sources: Historical Dictionary of American Slang; Oxford English Dictionary, 2nd Edition)