Within ninety days after the creation of any such paving district the governing body of the county shall cause an election to be held in such district at which shall be submitted to the qualified electors in the district the question whether the county auditor shall levy and the county treasurer shall collect an annual levy of taxes within the district, the number of mills thereof to be determined by the county auditor and stated in the resolution of the governing body of the county ordering the election, to raise a special fund sufficient over a period of years, not in excess of twenty years, to repay the cost of paving the streets and roadways in the district, but not less than the amount required to pay the principal of, and interest on, any county bonds issued under the provisions of this chapter to finance such paving.