I think that the use of the word 'bombproof' is intended more as slang toward any thick-walled structure that may be used for defensive purposes. This structure almost sounds like a dugout with gun ports aiming out in every direction. I don't knot how common they were but the logic seems to make sense.
Here is one example of 'bombproof being used in the vernacular of being an improvised defensive position. It comes from Horace Bell's 1881 book "Reminiscences of a Ranger" where he speaks of an incident where the Los Angeles Rangers (early Southern California militia) and some Vaqueros decided to prank the Mayor by staging a mock revolution in the middle of the night (ca. 1850s).
He describes the structure as follows.
"Intermediate between the plaza and Arcadia street, stood at that day the first monument of gringo enterprise, a brick culvert, which ran diagonally across the street and was about forty feet long, four feet wide at the base, and forming an arch, which was just high enough to admit a person in a low, stooping posture. Now that old culvert was a most infernal nuisance, being frequented by vagabond Indians as a place of convenience, which rendered the interior thereof unpleasantly odorous." (In other words, it was a place where drifters and vagabonds would go relieve themselves)
The Rangers put the Mayor into such a panic that he took shelter within this foul place as the Rangers stood outside of his refuge while laying down some covering fire.
"His honor (the Mayor) was safe, and the phalanx, dividing itself, took position at either end of the Mayor's bomb-proof, and opened a defiant fire on the exultant rebels, who now charged them on all sides."
In this case, the 'bomb proof' was a masonry-lined drainage tunnel.
-Dave