Early idiot boxes that weren't TV sets
A Google Books search for "idiot box" turns up an example from 1952—a bit earlier than the "circa 1955" that Merriam-Webster's Eleventh Collegiate Dictionary gives as the first occurrence date for the term in connection with television—that doesn't refer to TV at all. From Newsweek, volume 40 (1952) [combined snippets]:
In order to make the experiment applicable to other desert areas, scientists gauged the relative potency of Yuma [illegible] testing the men for stress and strain on a treadmill, dubbed "idiot box," set up first out of doors, then in a chamber capable of reproducing heat and moisture conditions of other deserts in the world [illegible]. After that the GI's marched with and without packs, dug foxholes, did tactical problems, and had such things as their pulses, weights, and temperatures measured scores of times a day.
A less-garbled account of this series of military experiments appears in The Leatherneck (1953):
Later, another [experiment] was made to learn the effects of heat on the minds and bodies of infantry troops in desert fighting, where the daytime temperature averaged 116 degrees. An experimental station, comparable to heat and humidity conditions of other deserts in the world, was set up at the Yuma Test Station in Arizona, where stress and strain on the men under intolerable heat could be gauged on a treadmill, which, housed in a hotbox, was promptly labeled "The Idiot Box." Eighteen human guinea pigs were put in relays on the squirrel run, with the heat well above 116 degrees. After a long session on the getting-nowhere hike, the men were taken out and urged on a double-time march, with and without packs, ordered to grub out foxholes, perform experimental tactical problems, during which time their pulses, temperatures were taken at regular intervals.
The American Mercury, volume 87 (1958) offers yet another account of this source of "idiot box."
"Idiot box" also seems to have been a popular term in aviation in the 1950s. From Sperryscope, issues 14–15 (1956):
Though extensive ground training, and about 10 hours in the "Idiot Boxes" (link trainers) had gone before, I had doubts of ever flying good basic instruments, for it requires a great deal of mental exercise and good co-ordination between eyeballs and the stick. But only three or four hops later, and as a result of the patient and constant effort of my instructor, I grew to enjoy the challenge of instrument flight.
From Society of Experimental Test Pilots, Quarterly Review (1957):
Even many experienced pilots find it difficult to operate a "flight simulator" using this display since the sensations of flight are completely absent. For this reason, flight simulators, in which the cockpit is firmly fixed to earth coordinates, have come to be known among jet pilots as "stimulators", "link strainers", "idiot boxes", etc. One pilot insists that the name "Flight Simulator" is a complete misnomer and that they should be known as "Procedural Trainers". Another states that when operating the so-called "Flight Simulator" using the artificial horizon
Additional mentions of "idiot boxes" in this sense appear in Aircraft, volume 19 (1957) and in Air Force Magazine, volume 41 (1958). In these instances, "idiot box" refers to the cockpit of a flight simulator. But the term carried over to a somewhat similar box used in a subsequent Air Force project for a fundamentally different purpose. From Virgil Grissom, "You Just Don't Have Time to Get Frightened," Life magazine (September 15, 1959):
The ability not to panic in a clutch situation could be very important in this mission [Project Mercury, the early U.S. space program]. When the doctors were testing us for final selection final selection, they had a device called the Idiot Box, which was really just a big instrument panel. It must have contained in one place everything the Air Force has ever done wrong in instrumentation. The instructions varied from instrument to instrument; there were handles to be turned or pulled, switches to be thrown, buttons to be pushed. Lights flashed at random all over the board in rapid sequence, and you had to put out the lights by hitting the proper switch or button. If you got the slightest bit behind, a horn blew loud right into your ear. It was supposed to rattle you, and it is significant that none of the final Astronauts let the horn throw him.
An Article in The Bee-Hive, volume 35 (1960) [combined snippets] throws further light on the nature of this "idiot box":
Two psychologists, Dr. Bryce O. Hartman and Captain Richard E. McKenzie, devised and built a machine which they called a complex behavior simulator. The candidates for Project Mercury named it the "idiot box."
"We built it all with a portable drill, a tool box, some salvage material, and a few dollars' worth of new items," explained Dr. Hartman.
The complex wiring was done with the help of a file clerk. The result is a panel which presents 32 separate problems on an array of instruments, lights, switches, knobs, and levers, each of which is operated manually. The problems are presented at increasing speed and constantly changing combinations, until it becomes impossible for the operator to control the panel. The object is to determine the limits of a man's ability to solve complex problems and make decisions. The best performances, by a considerable margin, have been made by men in training for the first space mission.
Early 'idiot X' television slang that didn't refer to TV sets
One interesting thing that J.E. Lighter, A Historical Dictionary of American Slang, volume 2 (1997) makes clear is that at least one and possibly two terms that use the wording "idiot X" arose as TV slang before "idiot box." Here is Lighter's entry for "idiot board":
idiot board n. IDIOT CARD 1 ["TV. a cue card"]; hence, the pane of a TelePrompTer. 1952 Newsweek (Aug. 4) 51: The Republicans and Democrats got their "idiot boards" free. 1955 Sat[urday] Eve[ning] Post (Sept 24) 29: "Idiot boards" are held out of camera range to prompt forgetful performers. Girls who hold them are called "idiot girls."
Though Lighter's first citation for "idiot card" itself isn't until 1957, "idiot board" is firmly attested by 1952, and "idiot girl" by 1955. The earliest Google Books match for "idiot board" in the relevant sense is from Twentieth Century Encyclopedia of Religious Knowledge (1951):
In certain parts of our country, as well as abroad, many thoughtful ministers read from the manuscript. Experts in television report that this form of public address does not "go over" unless the speaker uses an "idiot board," a device to permit reading without letting the people know that a person is so doing. On the whole the trend seems to be towards the method of our Lord, with prophets and apostles, who spoke from eye to eye, but never without orderly thought.
Additional matches appear in 1952 (in the Newsweek article that Lighter cites and in a New Yorker article) and in 1953 (in a play by Terence Rattigan and in Twenty-two Television Talks).
The earliest Google Books matches for "idiot cards" are from 1955, in History News and in International Stereotypers' and Electrotypers' Union Journal; and the earliest match for "idiot girls" in the relevant sense is the Saturday Evening Post article from 1955 noted by Lighter.
'Idiot box' as a TV set
The earliest Google Book match for "idiot box" in the sense of television set appears in Railroad Model Craftsman, volume 23 (1954) [combined snippets]:
How's your pike doing? Is it bogged down on incidentals and are operations somewhat on the shoddy side? Maybe you should merge your pike with that of a neighbor and see if that doesn't pep things up. It might mean the start of a new club. A neighborhood club doesn't necessarily have to have a club pike but can operate as a round robin with each member railroad being a host on a rotative basis. If you'll keep the Idiot's Box turned off and hide the comfortable chairs, chances are pretty good you'll get some work out of the guys.
As you can see, the term actually used here is "idiot's box"—but the apostrophe-s soon disappeared. A Google Books search finds instances of "idiot boxes," "Idiot Box," or "idiot box" beginning in 1958. From Alcoholics Anonymous, A.A. Grapevine (1958):
I have received a lot of pleasure from this work and have profited immensely, but yet my pocketbook is empty. So I will soon go out of the Indian world and back into the white man's world with his "stink-wagons" (automobiles), "devil boxes" (radio) and "idiot boxes" (television) and green stamps—plus the "overhead freight trains" (B-52's). There is no word for Alcoholics Anonymous in Indian so the boys adopted a literal translation — "The Whiskey Drinkers Club."
From Scottish Adult Education, issues 26–50 (1959) [snippet]:
What holds true for the theatre also holds true for the cinema and television. It was fashionable not so long ago to deride the idiot box, the goggle box, but television is now a part of our culture; just as much as painting or literature or music, it can enrich or impoverish our lives according to its content. We must not make the mistake of deriding the mass media but learn to use them without consideration of audience ratings in ways which will bring people together without standardising their responses.
From The Liguorian, volume 47 (1959) [combined snippets]:
Although a television set may occasionally be called an Idiot Box, it is in reality a Magic Box that can bring the whole world right into our living room. It's a crazy world, to be sure. Most of us would not be at home in it if it weren't. After all, television was never meant to supply for the Beatific Vision. Or, (to quote Browning) "What's a heaven for?"
Early newspaper matches for 'idiot box[es]' in the sense of 'TV'
An Elephind newspaper database search turns up several matches for the expression from the middle 1950s. From a letter to the editor of the [Urbana-Champaign, Illinois] Daily Illini (January 14, 1955) from Robert Nesbitt:
There is canned sex through records, movies, and cheap magazines, and as [Bob] Perlongo so aptly phrased it, "idiot boxes of sound and light" to afford us quick and easy diversion from life's realities. All these things are at the fingertips of the American in order that he may keep his mind in a pleasantly anesthetized condition.
From William Morris, "Words, Wit & Wisdom," in the San Bernardino [California] Sun (April 30, 1956), a discussion of recent novelties in "teen-talk":
Cigarettes are known as "chalk sticks" and television sets, in a phrase which many parents will wholeheartedly approve, are known as "idiot boxes." From the adult standpoint a rather less commendable, but wholly understandable, attitude is expressed by the slang expression describing a pupil who makes high grades on a "brain derby." He is said to have "pulled a skunk"!
And from a letter to the editor of the Aspen [Colorado] Daily Times (December 5, 1957) by Everett & Mary Millard:
One of the nicest things about Aspen is the absence of television. Those who have an "idiot box" in their home elsewhere, as we do, are lucky to find five percent of the programs worth seeing; yet people have the blame thing going all the time, to the detriment of their intellectual and social life.
"Idiot box" seems to have been used in military settings involving endurance experiments since at least 1952, and as a slang term for "link trainer" or "flight simulator" since at least 1956.
The first Google Books match for "idiot box" as slang for a television set is from 1954, but in that instance the term used is actually "idiot's box," "Idiot box[es]" in this sense appears in 1955 in a newspaper article in Illinois, and it proliferates after that. The term "idiot board" was in use at least as early as 1952 as television slang for "cue card."
The extent to which the earlier military and aviation senses of "idiot box" and the earlier television sense of "idiot board" may have influenced the slang term "idiot box" for "television set" is unknown to me; it seems quite possible that they had no connection to the coinage at all. William Morris, something of an authority on word and phrase origins, writes in 1956 that "idiot box" is "teen-talk," but I don't know whether his view that the expression originated as teen slang is widely accepted today.