From an FAA website: "May 1941.."Old Number One" was initially erected annually by the City of Miami to serve as the judge’s stand and timing tower for the "All American Air Maneuvers" held during the month of January. Due to a rapid increase in air traffic by 1941, the tower was left intact. Because of the increase, the Civil Aeronautics Board (CAB) ordered than an airport traffic control tower be commissioned at the Miami Municipal Airport or National and Pan American Airlines would no longer be authorized to operate passenger flight into Miami Municipal. The control tower with only light guns available to control traffic was commissioned on May 1, 1941. Radios were installed by the following month. The CAA assumed operation of the control tower on January 1, 1946."