The board shall designate an amount, not less than one-fortieth of the principal amount of the indebtedness, or series thereof, which shall be paid annually at a fixed time and place, together with the interest on all sums unpaid. The rate of interest shall be designated in the bonds and shall be paid semiannually at a rate not in excess of 8 percent per year; except that interest for the first year after the date of the bonds may be made payable at the end of said year.