When Jean Harlow, a platinum blonde, starred in Bombshell in 1933, the term bomb-shell had already been in use for at least 73 years (etymonline.com) in the sense of a "shattering or devastating thing or event." In most of her films, Harlow "was sluttish and smart, cracking gum and one-liners simultaneously: chewing up the scenery as the vulgar star of Bombshell" (Molly Haskell). It stands to reason to conclude that the blonde bombshell, first attested in 1942, was derived directly from Jean Harlow's persona. That, however, is not a felicitous answer to the question of why the filmmakers of Bombshell chose exactly that metaphor over another similar one. Wasn't, say, a volcano, likewise both an event and a thing, just as fitting a metaphor? Its erruption (~ explosion) is shattering. And devastating. Could the bombshell have been, simply, an arbitrary choice? Not likely.
It had been only 15 years since the end of the Great War, and the notion of sudden devastation by bombing was still omnipresent, what with all the unexploded bombshells strewn around England, not to mention the Western Front ("iron harvest"). Devastating and love (a common euphemism for passion) often go together and always have. And then, there was also the shape of the thing—the thing—which might've been an additional reason why not the more frequent word bomb was chosen, but rather the bombshell, the former having some, whether more or less, abstract realizations, and the latter certainly being more denotative of a thing, of an object, i.e., of the woman-object. The bomb was waved aside despite its not yet having become burdened by the secondary meanings of success, a failure, a marijuana cigarette, a large sum of money (all those meanings sprung up after 1933, according to OED); and despite its being synonymous with bombshell. Interesting. The shape, then. Well, a picture being worth a thousand words:
OK, two words: mammary, phallic.
Here it might be opportune to quote Wikipedia, "The term bombshell is a forerunner to the term "sex symbol" and originally used to describe popular female sex icons. Modern slang refers to a bombshell as an extremely sexually attractive woman."
At the peril of sounding like a feminist, I'll posit that this sort of labeling is a typical male thing. Wasn't the "bullet bra" ("torpedo bra") invented by a man? (Yes it was, by Howard Hughes, no less, in 1941.) That military terminology crept into the civilian life was, understandably, partly due to the vicissitudes of the times, but I think it has just as much to do with the old story of sex and violence. So there you have it.