Any county or municipality may levy on all of the taxable property of said county or said municipality, for the purpose of building and maintaining an airport either within or without the boundaries of such county or municipality, a tax not to exceed four hundredths percent (.04%) of market value for assessment purposes, on all taxable property within such county or such municipality, provided, however, that this section does not constitute a limitation upon the powers of cities as provided in section 50-321, Idaho Code.