Revenue bonds issued under the provisions of this article are declared to be issued for an essential public and governmental purpose and the principal and interest thereon, and income therefrom, shall be exempt from all taxes as provided in section 56-13. Such revenue bonds shall be made payable, as to both principal and interest, solely from the income, proceeds, revenues, and funds of Montgomery County derived from or held in connection with its undertaking and carrying out of urban renewal projects under this article; provided, however, that payment of such bonds, both as to principal and interest, may be further secured by a pledge of any loan, grant or contribution from the federal government or other source, in aid of any urban renewal projects of Montgomery County under this article, and by a mortgage on any such urban renewal projects or any part thereof, title to which is in Montgomery County. (Mont. Co. Code 1965, § 26-9; 1961, ch. 821, § 1.)