Whitman claimed that after years of competing for "the usual rewards", he determined to become a poet. He first experimented with a variety of popular literary genres which appealed to the cultural tastes of the period. As early as 1850, he began writing what would become Leaves of Grass, a collection of poetry which he would continue editing and revising until his death. Whitman intended to write a distinctly American epic and used free verse with a cadence based on the Bible. At the end of June 1855, Whitman surprised his brothers with the already-printed first edition of Leaves of Grass. Whitman paid for the publication of the first edition of Leaves of Grass himself. ( Wikipedia )
