bombshell (n.)

also bomb-shell, 1708, "mortar-thrown shell which explodes upon falling," from bomb (n.) + shell (n.).

BOMB, or BOMB SHELL, now called simply Shell (Fr. Bombe). A hollow iron ball or shell filled with gunpowder, having a vent or fuze-hole into which a fuzee is fitted to set the powder on fire after the shell is thrown out of a mortar. This destructive missile is intended to do injury both by its force in falling, and by bursting after it falls. [Arthur Young, "Nautical Dictionary," London, 1863]

The figurative sense of "shattering or devastating thing or event" is attested by 1859. In reference to a pretty woman "of startling vitality or physique" [OED], especially a blonde, it is attested by 1942. "Bombshell" as title of a movie starring blond U.S. actress Jean Harlow (1911-1937) is from 1933; it was believed to have been loosely based on the life of screen star Clara Bow.

The producers of the current hilarious Jean Harlow-Lee Tracy photoplay were not satisfied with the original title "Bombshell" so they renamed it "The Blond Bombshell." We wonder, in passing, why they didn't call it "The Private Life of Clara Bow" originally and let it go at that. [The Oklahoma News, Nov. 19, 1933]