Sponsored by the Skelly Oil Company, the Captain Midnight radio program was the creation of radio scripters Wilfred G. Moore and Robert M. Burtt, who had previously scored a success for Skelly with their boy pilot adventure serial The Air Adventures of Jimmie Allen.
Developed at the Blackett, Sample and Hummert advertising agency in Chicago, Captain Midnight began as a syndicated show in 1938, airing through the spring of 1940 on a few Midwest stations, including Chicago's WGN. In 1940, Ovaltine, a product of The Wander Company, took over sponsorship. With Pierre Andre as announcer, the series was then heard nationally on the Mutual Radio Network where it remained until 1942. It moved to the Merchandise Mart and the NBC Blue Network in September 1942. When the U.S. Government broke up the NBC Red and Blue Networks, Ovaltine moved the series back to Mutual, beginning September 1945, where it remained until December 1949.
The title character, originally Captain Jim "Red" Albright, was a World War I U.S. Army pilot. His Captain Midnight code name was given by a general who sent him on a high-risk mission from which he returned at the stroke of 12. When the show began in 1938, Albright was a private aviator who helped people, but his situation changed in 1940. When the show was taken over by Ovaltine, the origin story explained how Albright was recruited to head the Secret Squadron, an aviation-oriented paramilitary organization fighting sabotage and espionage during the period prior to the United States' entry into World War II. The Secret Squadron acted both within and outside the United States.
When the United States was attacked at Pearl Harbor, which curiously was foreshadowed in the program, the show shifted the Secret Squadron's duties to fight the more unconventional aspects of the war. Besides the stock villain, Ivan Shark, the war years introduced Axis villains Baron von Karp, Admiral Himakito and von Schrecker. The Secret Squadron wartime activities were usually outside the continental United States, with adventures in Europe, South America, the Pacific, and continental Asia. War-related subject matter included the theft of an experimental Flying Wing aircraft, radar coupled antiaircraft guns, jet aircraft and other weapons.
After the war, some of the newer villains used war surplus equipment to carry out their activities. Secret Squadron activities shifted to contending with criminals as well as spies. The action continued to operate internationally, with adventures in South America and Africa as well as within the United States.
The show was extremely popular, with an audience in the millions. Just under half the listeners were adult, and it was a favorite of World War II United States Army Air Force) crews when they were stationed in the U.S. Radio premiums offered by the series (usually marked with Midnight's personal symbol of a winged clock with the hands pointing to midnight) included decoders. These Code-O-Graphs were used by listeners to decipher encrypted messages previewing the next day's episode, usually broadcast once a week. Other premiums included rings, telescopes, and World War II items. (The broadcast messages were encrypted with relatively trivial monoalphabetic substitution ciphers with word division.)
The scripts depicted women who were treated as equals, not just characters waiting to be rescued. Both Joyce Ryan of the Secret Squadron and Fury Shark, daughter of villain Ivan Shark, pulled their own weight in the adventures. Joyce went on commando raids and became involved in aerial dogfights during World War II.
- Captain Midnight – World War I aviator who leads the Secret Squadron, though he spends much time in the field actively contending with crime, espionage, and sabotage. He is extremely skilled aviator with an ability to fly almost any aircraft superlatively. The Captain is usually accompanied by a team consisting of three Secret Squadron members (played by Ed Prentiss, Bill Bouchey and Paul Barnes, with Prentiss playing his part the longest).
- Chuck Ramsay – Captain Midnight's ward, a young man in his late teens or early twenties who is a Secret Squadron agent. Prior to the formation of the Secret Squadron, he shared adventures with his guardian. A member of Captain Midnight's usual team, he was played by Jules Getlin, Dolph Nelson, Billy Rose, Jack Bivans and Johnny Coons.
- Ichabod "Ikky" Mudd – The Secret Squadron's Chief Mechanic. Mudd knew Captain Midnight briefly before the Secret Squadron was formed, and joined the Squadron shortly after it was formed. A member of Captain Midnight's usual team played by Hugh Studebaker, Sherman Marks and Art Hern, he was responsible for the development of the Code-O-Graph and also developed some weapons before and during World war II.
- Joyce Ryan – A young woman in her late teens or early twenties who is a Secret Squadron agent. She was originally discovered as an amnesiac by Captain Midnight and Chuck Ramsay during a 1941 skirmish with the forces of Ivan Shark. She became a Secret Squadron member after several adventures with Captain Midnight, Chuck Ramsay, and Ichabod Mudd, replacing an earlier female companion named Patsy Donovan. Prior to World War II, she regained her memory and elected to remain in the Secret Squadron. A member of Captain Midnight's usual team, she was played by Marilou Neumayer and Angeline Orr.
- Agent Kelly, SS-11 – Lyle William Kelly, a Secret Squadron agent who frequently accompanied Captain Midnight's usual team on their adventures. Kelly was Captain Midnight's usual liaison to his superior, Major Barry Steele. Kelly was played by Olan Soule.
- Major Barry Steele – U.S. Army Intelligence officer recalled from inactive duty as Captain Midnight's superior officer. Steele usually worked in the background but often provided the Secret Squadron with data and equipment. He often provided assignments for the Secret Squadron but left its administration to Captain Midnight.
- "Mr. Jones" – Pseudonym used by the mysterious highly placed government official who created the Secret Squadron and made Captain Midnight its commander, implicitly the President of the United States.
- Ivan Shark – Ruthless criminal mastermind who developed a highly efficient mercenary organization. Shark often sold services of his organization to agents of foreign governments. Played by Boris Aplon, he was the default villain on the radio program. Although he was captured or thought killed many times, he always returned to plague the hero until, in the final episode, he was eaten by a polar bear while Captain Midnight watched from a plane overhead.
- Fury Shark – Ivan Shark's devoted daughter and sadistic second in command in his organization. She frequently took command when Ivan Shark was unable to function (e.g., in prison). Highly intelligent, she always proved herself capable. She was played by Rene Rodier and Sharon Grainger.
- Gardo – One of Ivan Shark's principal aides. Gardo was portrayed as being loyal but relatively slow witted, and frequently the butt of Shark's wrath. Occasionally acted as Ivan Shark's pilot.
- Fang – One of Ivan Shark's pre-war aides, Fang was an Asian, who always addressed Shark as "Master." His presence vanished after World War II.
- The Barracuda – Head of an extensive criminal organization similar to Ivan Shark's, but headquartered in the Orient. The Barracuda headed the Tiger Tong, a Chinese gang, and had his own private air force. Killed in 1942.
- Baron von Karp – Nazi World War II villain with whom the Secret Squadron contended, first in the United States, then in occupied Europe.
- Admiral Himakito – Japanese officer with whom the Secret Squadron contended in the Pacific theater during World War II.