When said board of public utilities shall find it necessary and expedient to have the city vote bonds for the construction, extension or improvement of the municipal airport and municipal fields for aviation purposes, it shall be the duty of the mayor of said city, within 30 days after receiving written request from said board of public utilities to issue a proclamation for holding an election to vote bonds in the amount requested by the board of public utilities according to law. If such bonds carry by a legal majority, the governing body of the city shall then issue such bonds in the amounts requested by the board of public utilities and deliver the proceeds thereof to the treasurer of the board of public utilities, provided that if bonds have been authorized by an election but not issued, such bonds may be issued at any subsequent time, at the request of the board of public utilities.