The corporate authorities may borrow money on the credit of the corporation for corporate purposes, and issue bonds therefor, in such amounts and form, and on such conditions unless otherwise provided in this Code as the corporate authorities prescribe. Before, or at the time of incurring any indebtedness, the corporate authorities shall provide for the collection of a direct annual tax sufficient to pay the interest on the debt as it falls due, and also to pay and discharge the principal thereof within 20 years after contracting the debt.
65 ILCS 5/8-1-3