The board or boards of supervisors of the county or counties in which the district lies shall, at the time of fixing the general tax levy, sometimes called the annual assessment or regular annual assessment, for such district, and in the manner for such general tax levy provided, levy and collect annually each year until said bonds are paid or until there shall be a sum in the treasury set apart for that purpose sufficient to meet all sums coming due for the principal and interest on such bonds, a tax sufficient to pay the interest on such bonds as the same becomes due and also, to constitute a sinking fund for the payment of the principal thereof at maturity. The sum for the sinking fund shall in any event be sufficient to provide for the payment of the principal of all of the bonds as such bonds become due. Said tax shall be in addition to all other taxes levied for district purposes and shall be placed in the bond interest and sinking fund of the district and, until all of the principal and interest of the bonds of said district is paid, the moneys in said fund shall be used for no other purpose than the payment of said bonds and accruing interest thereon.