The board of directors shall in each year before said assessment roll for any general improvement or divisional district herein authorized, is delivered to the respective county treasurers, levy an assessment sufficient to raise the ensuing annual interest on the outstanding bonds issued for the benefit of said district, and shall beginning in the year preceding the maturity of any series of the bonds of any issue, levy an assessment for the ensuing year and from year to year in an amount sufficient to pay and discharge said outstanding bonds as they mature.