The board may determine and fix a date for the earliest maturity of the principal of the bonds, or series thereof, not more than 10 years from the date of issue of bonds. The principal amount of the indebtedness, or series thereof, must be made payable in not to exceed 40 years from the time of contracting the same; and beginning with the date of earliest maturity of the bonds, or series thereof, not less than one-fortieth of the principal amount of the indebtedness, or series thereof, shall be paid annually.