In early 1930, Street & Smith Publications hired David Chrisman and Bill Sweets to adapt the Detective Story Magazine to radio format. Chrisman and Sweets felt the program should be introduced by a mysterious storyteller. A young scriptwriter, Harry Charlot, suggested the name of "The Shadow." Thus, "The Shadow" premiered over CBS airwaves on July 31, 1930, as the host of the Detective Story Hour, narrating "tales of mystery and suspense from the pages of the premier detective fiction magazine." The narrator was first voiced by James LaCurto, but became a national sensation when radio veteran Frank Readick, Jr. assumed the role and gave it "a hauntingly sibilant quality that thrilled radio listeners."
The series disappeared from CBS airwaves on March 27, 1935, due to Street & Smith's insistence that the radio storyteller be completely replaced by the master crime-fighter described in Walter B. Gibson's ongoing pulps. But on September 26, 1937, The Shadow radio drama premiered with the story "The Deathhouse Rescue", in which The Shadow was characterized as having "the power to cloud men's minds so they cannot see him." As in the magazine stories, The Shadow was not given the literal ability to become invisible. Thus began the "official" radio drama, with 22-year-old Orson Welles starring as Lamont Cranston, a "wealthy young man about town." Once The Shadow joined Mutual as a half-hour series on Sunday evenings, the program did not leave the air until December 26, 1954.
Welles did not speak the signature line, "Who knows what evil lurks in the hearts of men?" Instead, Readick did, using a water glass next to his mouth for the echo effect. The famous catch phrase was accompanied by the strains of an excerpt from Opus 31 of the Camille Saint-Saëns classical composition, Le Rouet d'Omphale.
After Welles departed the show in 1938, Bill Johnstone was chosen to replace him and voiced the character for five seasons. Following Johnstone's departure, The Shadow was portrayed by such actors as Bret Morrison (the longest tenure, with 10 years in two separate runs), John Archer, and Steve Courtleigh. (Source: wikipedia.org)