Hate to burst a bubble on this, Alex, but my old friend, Joe Niagra, started out playing records on the radio back about 1937--Mark will remember him from WIBJ--he was one of the early hosts of American Bandstand (when it was on the radio) and was one of the DJ's caught up in the infamous "payola" scandals--

As to the "Top 40" theory, I kind of doubt it--the top 40 format was originally conceived as a tight format that was intended to keep the audience's attention by play hits and only hits, based roughly on the Billboard charts, and those have existed since 1913(though the number of records shown has varied at different times)

I was a very small time DJ back in the late 60's,and we picked our records according to a series of rotation formulae that featured a top 5, a top 20, and a second 20--different times of the day, you could play a different proportion of things from each--the late night guy could play album cuts and even got to choose his own stuff--

The idea that the top 40 evolved from the number of records you could fit into 2 hours makes sense--I think that the top 40 format didn't really get started til after the payola scandals, where a lot of top top DJ's turned out to be taking money in exchange for playing records, so the stations wanted more control over what was played--