The state of North Dakota covenants with the purchasers and with all subsequent holders and transferees of bonds or notes issued by an authority pursuant to this chapter, in consideration of the acceptance of and payment for the bonds or notes, that the bonds and notes of an authority issued pursuant to this chapter and the income therefrom, and all moneys, funds, and revenues pledged to pay or secure the payment of such bonds or notes shall at all times be free from taxation, except for estate taxes and taxes on transfers by or in contemplation of death.
N.D.C.C. § 40-61-15