We all agree (hopefully) that the Golden Age of Comics began with Action Comics #1 (Ju'38) with Superman on the cover. And we all know that Superman didn't reappear on the cover until #7 (D'38). When I was researching one of those darn Twenty Questions, I was amazed to learn that in the same month, Detective Comics #22 featured the Crimson Avenger, a costumed hero albeit a non-powered one.
When Action Comics #12 (Jl'39) with its cover highlighting both Superman and Zatara, Detective Comics #27, of course, had the debut of Batman!
But it took Superman a few more months to be on every cover of Action Comics even with Superman #1 coming out in 1939. Nor was Batman on every cover of Detective during his first year! His second cover was in Detective #29 (Jl'39), the same time as the Sandman's debut in Adventure Comics #40.
By September 1939, Superman, Batman and Sandman were cover featured though the Sandman was not consistently promoted. All American Comics featured its early "superhero" with Gary Concord, Ultra-Man and in January 1940, Flash Comics arrived though it took several months until the Flash and Hawkman were the rotating cover stars.
The Spectre gave More Fun Comics a regular cover feature though he would share it with Doctor Fate and the Sandman got pushed off by Hourman in Adventure #48. When Green Lantern debuted in All American Comics #16 (Jl'40), all the books had super-heroes on their covers regularly, some two years after Action Comics #1.
So while the Golden Age began in 1938, it didn't fully thrive until 1940.This early expansion would culminate with All Star Comics #3 at the end of 1940.
I can think of one reason Crimson Avenger might have started sporting longjohns -- because originally he was such a ripoff of Green Hornet that it was painfully obvious. Both were newspaper publishers in their civilian life, both wore similar outfits, both used gas guns. Oh, and both had Asian sidekicks.