In Dangerously, (pages 186-187) it states: The Monstrous gorilla had been gestating in Cooper's imagination since 1929, but after John Hambelton's death Cooper had focused on the emerging storyline with a new passion. And in February 1930, when he was leaving his midtown office, heard an engine overhead, and looked up as an airplane, gleaming in the setting sun, passed over the New York Insurance Building. "Without any conscious effort or thought I immediately saw in my minds eye a giant gorilla on top of the building."

Well I for one am glad that a plane was flying overhead the moment Cooper stepped out into the street.