Justin Bieber -- I Wanted to Bang Selena Gomez Since I Was 15 Years Old

Justin Bieber hatched his plan to nail Selena Gomez 2 years before they dated -- while she was still dating a Jonas brother -- and TMZ has the footage that proves it.

A 15-year-old Bieber confessed the scheme back in July 2009 -- during an interview with L.A. radio host DJ Skee that was lost in oblivion -- saying his celebrity crush was Selena Gomez ... who was dating Nick Jonas at the time.

Bieber immediately blushes like a schoolgirl and says, "Nobody heard that" -- but less than two years later he attended a Vanity Fair Oscar party with the girl of his dreams.