In the original iteration of Batman's origin story, published in Detective Comics No#23 in Novembers of 1939.
The Legend of The Batman and how he came to be
The origin of Bruce getting the idea to become a Bat was him sitting in his study knowing he needs a disguise and trying to think of one, saying "Criminals are a superstitious and cowardly lot" so determines it should be a scary disguise.
Then a bat flies in through the window and he goes "A Bat!That's it! It's an Omen. I shall become a BAT!"
This scene has never been dramatized in Live Action. And even in
Animation only in the adaptation of Miller's Year One that adds unneeded
additional drama with Bruce bleeding out and planning to kill
himself if he doesn't see some sort of sign.
In the 80s Denny O'Niel had introduced an alternative explanation of
a Bruce as a kid falling into a Cave with Bats. Both Batman Forever
and Batman Begins borrowed this. But Forever made it a giant Bat-Demon,
which I felt was a DKR Homage.
The BTAS continuity establishes in Mask of The Phantasm him getting the ideas from a swarm of Bat's cockblocking him.
I remember on Smallville's IMDB board we had a lot of fun giving our
own ideas on what the very last shot of Smallville should be. Clark
ripping his shirt open to reveal an S on his chest was a popular
I think, IF this show manages to last 10 seasons and end with Bruce
20 or older. That Iconic scene should be how it ends. I really want to
see it in Live Action, out-dated dialogue and all.