W1XOJ was the first FM radio station, granted a construction permit by the FCC. On January 5, 1940 FM radio was demonstrated to the FCC for the first time. FM radio was assigned the 42 to 50 MHz band of the spectrum in 1940. FM radio, and later stereo FM radio, were both developed in the United States primarily by Edwin Armstrong.
Source: Wikipedia.

Happy Birthday FM Broadcasting.

FM Stereo? That sounds very unlike Major Armstrong!

Perhaps not stereo... but multiplex:

http://users.erols.com/oldradio/eha71.htm#Arm

A long time ago, I remember reading part of Armstrong's IEEE paper, in which he postulated that a frequency modulated subcarrier could be added to the main program audio feed to a FM transmitter to carry a second channel of audio. I wasn't aware that it took until 1952 to achieve this.

The transmitter exciter described in Armstrong's paper was an indirect FM system, where the oscillator ran at 50 kHz. This was phase modulated by passing it through a balanced modulator, and adding the modulator output to the same carrier with a 90 degree phase shift. This combined signal was passed through a limiter and then through a long frequency multiplier chain. With the first stage operating at such a low frequency, it would have been difficult to get a bandwidth greater than that required for the baseband audio.

I've seen FM Stereo variously attributed to Tony Csicsatka (General Electric) and Carl G. Eilers (Zenith).

Jeffrey is correct. There were 14 battling proponents and the GE and Zenith systems were combined to what was accepted by the FCC.

The wikipedia history is accurate.

Sidenote: If you scroll down the wiki entry linked you can read about the history of quadrature. In the early 70's while I was studying for my EE, my lab partner and I (mostly my lab partner, genius that he is) developed a quad system that the professor liked so much he had us make patent application to the schools patent attorney whom eventually came back to us and said our idea was too much like the one that RCA had just licensed from Louis Dorren and had/would become the FCC approved method.

Sigh.

Sigh Star.

If Armstrong did indeed invent multiplex, my guess is that he wouldn't have been happy enough with the results to try and gain acceptance for the system. Either that or it was much more effective at 42-50mc. (I understand there would be less multipath interference.) Since stereo recordings weren't available in any kind of numbers until 1958, I have to wonder what he had in mind back then.

Live stereo performances of orchestra's. That's how stereo was first broadcast using AM & FM stations for channeled stereo in homes.

"I have to wonder what he had in mind back then."

Perhaps he was interested in broadcasting 2 discrete channels for other reasons, like music on the main channel and news on the sub channel?

However, by the time of his Multiplex experiment, stereo recordings had been made, and this would seem to be an obvious desired extension of his higher quality broadcasting technology.

http://history.sandiego.edu/GEN/recording/stereo.html