Motion Picture Herald, May-June 1940.
The Film Daily, April-June 1941
Motion Picture Herald, May-June 1943.
These three trade publication notices (click to enlarge) each reference a different unmade Columbia serial. On Ebay, over ten years ago, I actually stumbled on a cache of Columbia poster art by the studio’s ace artist Glenn Cravath which contained pretty finished-looking poster art for all three of these titles, which is how I was knew which names to search for in the trade publications. In an earlier version of this post, I tried to describe this art from memory, but I’ve recently managed to locate it in the depths of my image archives. Thus, I’ve prepared a new edition of the post, with color pictures this time.
It’s interesting to speculate on why these three announced titles never actually got made. Holt of the Secret Service replaced Daniel Boone on the Columbia release slate, and I can envisage the hypothetical reasons for the switch. Holt was a Columbia contract player about to exit the studio, and a one-time major star with some remaining name value for older moviegoers; Columbia obviously decided to stick him in a serial to draw in some additional adult viewers, and likely decided that they’d have him play a character with his own name–allowing them to use it in the title and boost the serial’s promotional value further. They couldn’t just stick him in one of the announced serials, however; White Eagle was already a lock for Buck Jones, since it was a “remake” of a successful Columbia B-western starring Buck Jones; the Spider sequel was likewise a lock for Warren Hull; The Iron Claw was a “mystery” serial built around its villain and not a suitable star vehicle for a leading man; Adventures of Daniel Boone was built on a famous historical hero, not an original character who could be named “Holt.” With Jones already on board for White Eagle, the Spider sequel doubtless being regarded as a surefire winner given the success of its predecessor, and the Iron Claw’s mystery-villain element probably being considered highly exploitable, it seems logical that Daniel Boone would draw the short straw and be bumped from the schedule to make room for the Holt vehicle.
It’s easier to guess why Pirate Gold was dropped, since its place was taken by The Secret Code. From its title and advertising art, Pirate Gold was a non-topical, non-political treasure hunt serial, and was doubtless axed to make room for Code, Columbia’s first full-fledged World-War-2-themed serial, following Pearl Harbor. As for The Fighting General, I suspect that it was announced when Columbia was still in a hurry to capitalize on the war, and then abandoned when someone, either at Columbia or at their serial-producing Darmour Productions affiliate, realized that the premise was basically unworkable for a serial; to quote Irving Berlin, what can you do with a general? There seems to be no way that a high-ranking military officer could be plausibly inserted into the one-on-one fistfights, chases, and other action scenes required of a serial star, or that an officer of that rank could be believably played by someone of the typical serial star’s age. General’s place was ultimately taken by The Desert Hawk, which was also built on an offbeat premise but one better-suited to translation to the serial milieu.
The Files of Jerry Blake 
Nice update. I think your reasoning as to why the three serials mentioned were never made rings true. Even at first glance, the “Fighting General” concept seems pretty unworkable, especially given the command structure of modern-day warfare. Daniel Boone’s life and exploits have been greatly misconstrued in their film and television adaptations, and I have no doubt this serial would have just been one more entry in that pattern, so no big loss. “Pirate Gold” is the only one, if done right, that might have held some promise. However, given Columbia’s generally lackluster track record, it probably wouldn’t have amounted to much, so substituting a more timely wartime narrative definitely made sense.
Speaking of Daniel Boone, there’s a very well researched biography of him that was written by John Mack Faragher. It goes a long way toward dispelling the myths that have developed over the years, and presents a Boone that is a man of his times and a much more interesting character than any fictionalized version could hope to be.
(this comment from the previous edition of this post was lost when it was replaced; through the Internet Archive and the efforts of Michael Litant, it’s been revived).