Han Solo first appeared in the rough and first drafts of Star Wars as a Jedi warrior and friend of General Skywalker. As the story progressed, Solo became a fat, bearded Corellian pirate who dressed flamboyantly (think classic Blackbeard). By the third draft, Solo became the reckless starpilot we all know and love.
In the National Public Radio Dramatizations of the Star Wars trilogy, Han was played by actor Perry King.
Solo's pre Star Wars adventures were explored by Brian Daley in his trilogy of novels: Han Solo at Star's End, Han Solo's Revenge, and Han Solo and the Lost Legacy, published by Del Rey Books in 1979-1980. Young Han's life was expanded in the late 90s in a trilogy of novels by A.C. Crispin: The Paradise Snare, The Hutt Gambit, and Rebel Dawn, published by Bantam Books.
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Solo joined the Imperial Academy as a teenager, to refine his piloting skills before turning to the smuggler's trade.
Despite being a great pilot, Solo was drummed out of the service when he defied orders and rescued a Wookiee slave, Chewbacca. Since that day Chewbacca has owed a life debt to Solo and hasa remained his steadfast companion.
His military career in shambles (he freed a prisoner and efied orders, treason and mutiny), Solo returned to the fringes, joining a smuggling gang. It was here that Solo had his first Kessel Run. He traveled with such notorious fringers as Shug Ninx, Mako Spince, Salla Zend, Rik Duel and Lando Calrissian. (some of these contacts are described in various expanded universe stories including the Marvel Comics series and Dark Horse comics series.
After winning the Millennium Falcon from Calrissian in a cuthroat game of sabbac (which ruined their friendship untill The Return of the Jedi, Solo and Chewie worked in the Corporate Sector for some time, for crime-bosses. Through these connections he ended up in the employ of Jabba the Hut.
info from StarWars.com, hasbro.com and various books and comics from marvel comics and dark horse comics.