You think you know Batman? The Caped Crusader's mysteries are much deeper than you've ever fathomed. Over 60-odd years, in an assortment of media, Batman has gone in some incredibly weird directions. Here are 10 Bat-facts you didn't know.

1) Batman only met Hitler once.
At least during World War II. While other superheroes were busy punching Hitler every other day, Batman barely fought Nazis at all. In one comic, Nazi saboteurs come to Gotham City and Batman does take them down. But Batman's only confrontation with Adolf Hitler himself happens on the cover of World's Finest #9 — where Batman and his friends throw tennis balls at Der Fuhrer.
Source: Batman Unmasked: Analyzing A Cultural Icon by Will Brooker.

2) Robin was originally planned to appear in one issue, and then possibly disappear forever.
Originally, creator Bob Kane wanted to try out Robin in one issue, but Bat-editor Jack Liebowitz was against the idea of having a kid fighting gangsters, because "Batman was doing well enough by himself." But after Detective #38 hit the stands with Robin in it, the issue sold double what the issues with just Batman had sold. So Liebowitz sheepishly agreed to keep Robin in future issues.
Source: Batman Unmasked: Analyzing a Cultural Icon by Will Brooker.

3) Some of Batman's biggest villains were originally more animalistic.
Catwoman originally wore a furry cat mask that covered her whole face. And the Penguin was actually based on the cartoon penguin who advertised Kool menthol cigarettes in print, and also appeared on the radio shouting, "Smoke Kooools!" "Smoke Koools!" The original version of the Penguin was way more cartoony and hyperactive.
Random unrelated fact: Bob Kane and Bill Finger used to sit in Poe Park, in front of Edgar Allan Poe's single-frame white farmhouse in what's now the Bronx, and brainstorm Batman story ideas.
Source: Batman: The Complete History by Les Daniels, Tales of the Dark Knight, Mark Cotta Vaz.

4) Batman used to kill criminals all the time.
He hurled them off rooftops — including one member of the Frenchy Blake gang whom he tossed with "a mighty heave." In the first Bat-story, "The Chemical Syndicate," he punched a criminal into a vat of acid, and says, "A fitting end for his kind." He also strangled criminals to death with his lasso, kicked them so hard he broke their necks, and punched them so hard they fell to their deaths. In Batman #1, he kills a bunch of Hugo Strange's henchmen with a machine gun. "Much as I hate to take human life, I'm afraid this time it's necessary!" Batman shouts.
Source: Tales of the Dark Knight, Mark Cotta Vaz, Batman Unmasked by Will Brooker.

5) Batman: The Dark Knight Returns by Frank Miller got horrible reviews.
At least, the 1987 New York Times review of this seminal Batman graphic novel, by Mordecai Richler, is pretty damning:

The stories are convoluted, difficult to follow and crammed with far too much text. The drawings offer a grotesquely muscle-bound Batman and Superman, not the lovable champions of old... If this book is meant for kids, I doubt that they will be pleased. If it is aimed at adults, they are not the sort I want to drink with.

Here's a pretty hilarious collection of damning reviews of TDKR from, too.

6) A lot of key elements of the Bat-mythos weren't introduced until the 1940s.
Batman's home was first named as "Gotham City" in Detective Comics #48 in 1940, and before that Batman lived in "Metropolis" or just New York. The Bat-signal didn't appear until Detective Comics #60 in 1942. The Batcave didn't appear until 1948, and prior to that Batman just had a secret hangar for the Batplane and a deserted barn connected to Wayne Manor by a secret passageway. Prior to the appearance of the Bat-signal, Batman covers frequently depicted a giant moon (which was frequently bright yellow) behind Batman.
Source: The Golden Age of Batman by Artabras

7) The crappiest Batman costume ever sold was part of the carton of Batman (TM) chocolate milk. You cut the side of the milk carton out and you end up with a soggy piece of cardboard shaped sort of like a Bat eye-mask. Lord knows what that would smell like after a few hours. Here's a similar cut-out mask from another milk carton that wasn't actually called "Batman Chocolate Milk" but seemed to have licensed the same design. Want to see some other weird Bat-mercandise? Check out a Bat-troll doll, a strange masturbating Batman figure, and Russian Batman.
Source: Batman Collected by Chip Kidd and Geoff Spear

8) Gotham City was almost located in California.
In real life, not in the comics. Sunnyvale, CA attorney Joseph F. Lewis owned a nightclub in the Silicon Valley town, and he renamed it from Whiskey-A-Go-Go to Wayne Manor in 1966, during the height of Bat-mania. The newly renamed nightclub had Batman-themed decor and dancers dressed as Batgirl and Catwoman. Eager to revitalize the town's downtown, Lewis lobbied the city council to rename the city from "Sunnyvale" to "Gotham City." And he nearly pulled it off.

9) Barbara Gordon only became Batgirl because the television show demanded it.
The producer of the late-1960s television series, William Dozier, requested that the comic add a new female counterpart to Batman, because Betty Kane, the former Batwoman, was long gone. The television show needed a new female character, and they wanted the comics to keep step. So Commissioner Gordon's librarian daughter Barbara suited up as Batgirl for the first time in 1967. Alfred, Bruce Wayne's butler, was also dead in the comics — but the comics creators brought him back to life at Dozier's request. Alfred returned in one of Batman's weirdest stories, in which the supernatural mystery villain, the Outsider, turned out to be Alfred in disguise.

10) Batman fought Dr. Doom before the Fantastic Four ever did.
This happened in Detective Comics #158 (April 1950). When Batman and Robin are bringing their 1001st trophy into the Batcave, Dr. Doom smuggles himself inside, so he can get inside their secret lair. Dr. Doom then rigs all the other Bat-trophies to kill the Dynamic Duo, only to die himself. (I could have sworn I came across an early-1950s Bat-villain called Dr. Evil in my researches, too, but now I can't find him again.) There was also a recurring Bat-villain named Dr. No-Face, who's curiously close to the James Bond villain in name.

This io9 Flashback originally ran February 2011.