Bonds issued are obligations of the district and the board of supervisors of the county shall, at the time of fixing the general tax levy and in the manner provided for the general tax levy, levy and collect annually until the bonds are paid, or until there is a sufficient sum in the treasury of the county set apart to meet all sums coming due for principal and interest on the bonds, a tax sufficient to pay the annual interest on the bonds and the part of the principal which will become due before the time for fixing the next general tax levy.