Whenever the city engineer of any city of the second class has filed with the governing body of such city as master plan for the physical development of said city, which plan shall be limited to public improvements within said city, and may provide for assumption of all outstanding indebtedness of benefit districts heretofore created for public improvements, and, also, for refunding payments made on assessments when the outstanding indebtedness of a benefit district is assumed by said city; and which plan will require a number of years to execute, and such master plan is approved by the governing body of said city, such city is hereby authorized to issue general obligation bonds in an amount sufficient to carry out such a master plan.