The mistake is buying a bomber a size up because the ribbing looks like it'll swallow anything. It won't. That ribbing at the hem and cuffs is doing a very precise job, and if the body of the jacket is too big, you end up with a sort of pillowcase shape, wide at the middle and snapped in at the bottom, which flatters nobody and ages the jacket by about a decade before you've worn it out of the shop.
This is not a jacket for anything that needs to look remotely formal, and it's not much good in real rain either, most of them are showerproof at best and the ribbing holds water like a sponge once it's properly wet. It also sits at an odd length for a lot of dresses, cropping right at the point where a dress wants to flow, so if your wardrobe leans towards floaty midi dresses, a bomber will fight that silhouette rather than finish it.
Fit is everything here, more than with most jackets. You want it close through the shoulders, a bomber with dropped or baggy shoulders just looks like it's wearing you. The body should skim rather than cling, with enough room to layer a jumper underneath in winter, but the ribbed hem needs to sit at or just below your natural waist. Too long in the body and it bunches over the ribbing like a sausage roll, too short and it rides up every time you lift your arms, which is often, because you will be reaching for things all day.
In the shop, check the zip pull is metal and moves properly, cheap plastic ones snag within a month. Check the lining, a bomber with no lining at all will feel clammy and cling to anything you wear under it. Feel the ribbing itself, it should have a bit of spring to it, if it's gone slack straight off the hanger it'll be baggy at the wrists within a few wears. And check the pockets sit flat when empty, a bomber with pockets that gape open is a bomber that'll look untidy by lunchtime.
It clashes badly with anything already boxy, a wide leg trouser and a bomber together just reads as a lot of fabric with no shape anywhere, and it sits uneasily with anything too delicate, lace or silk underneath looks like it's wandered in from a different outfit entirely. Wash it according to the label and not by instinct, the ribbing is the part that goes first and a hot wash will shrink it out of all proportion to the rest of the jacket.