When were radio plays invented?

Radio plays go back to the very earliest years of broadcasting, in 1921-1922. Because all radio was live (tape had not yet been invented), stations needed something to fill up the time. So, a lot of singers, musicians, poets, professors, and comedians went on the air. But some stations also began broadcasting "radio dramas," which were plays written to be performed on the radio. Thus, it's not so much that radio plays were invented as much as they were popularized-- some stations used already famous plays (like works of Shakespeare) and others hired writers to create short and interesting dramatic works that would hold the audience's attention. Some stations were able to persuade experienced actors and actresses to go on the air and play some of the roles; other stations used local amateurs, or even staff members from the radio station. And every radio station had a man or a woman with expertise in "production"-- creating the sound effects necessary to make the play sound realistic.