From the Niagara Falls Review:
The Rainbow Bridge, which has become one of Niagara Falls’ most recognizable structures, was built by the Niagara Falls Bridge Commission. Made up of members from both Canada and the United States, the Commission still owns and operates the Rainbow Bridge, as well as the Whirlpool Rapids Bridge and the Queenston-Lewiston Bridge.
Formal groundbreaking ceremonies for the Rainbow Bridge took place on the American side, May 16, 1940.