Hey guys, I'm looking to DM a 1930s DnD campaign. It's going to be a lot of work reskinning everything and making it work. I'm not married to Dungeons & Dragons, so I was wondering if you had any suggestions on a better existing system for 1930s.

It can have magic or not, sci-fi technology or not. Honestly, I can work with anything that starts be in the 1920s-1940s without having to manually recreate DnD myself.

If it helps, the story will focus on a single city, a criminal mob (and fights with their goons), political corruption, detective work, and some other goofy and fun stuff along the way.

Thanks for anything you guys can suggest!

There is a little-known game called Call of Cthulhu which happens to be set in the 1920s. I recommend it. The game is setup for existential horror but in fact the core system (BRP) is generic and well-suited to lots of genres. If you want a more pulp feel, there is Pulp Cthulhu.

Call of Cthulhu, or take its generic core system only, BRP.

I'd suggest Blades in the Dark, just ignore the lore/setting.

Okay so...why default to dnd if you aren't doing fantasy? Like...what genre are you aiming for? Just like...noir? Try gumshoe maybe? Call of cthulhu is default that time period but it's more investigative horror. Though i have used it for non horror noir before

The default early last century system is call of Cthulhu. It has a pretty solid beginner set with good adventures. It’s got magic but not high fantasy magic.

If you want magic or sci fi you’ll get into more obscure games, but I’d ask yourself if you want that specifically.

If you don't have experience outside of d&d, I highly recommend trying mytants and masterminds 3e. It uses a modofied d20 system so the transition should be comparaty easy and has support for the "golden age" of comics; the 30s and 40s. It supports magic and supertech easily so you can season access to powers to taste, there's even a fanmade "monster manual" (I forget exactly what they called it) but it adapt characters and critters from all over pop culture that can be reskinned in all of 2 seconds as-needed.

I mean Sharn from dnd's eberron is a pretty good setting.
Focus on a single city, criminal mob+goons(clans Boromar and Daask) political corruption(city watch is famously corrupt, houses are super corrupt), detectives and noir are associated with Sharn. And the best part is, everyone is so grey that any given faction can be a protagonist or antagonist faction.

A mob game in the 30s? See if you can get a copy of Gangbusters off DriveThru or eBay. Thats literally what that game was built for.

Call of Cthulhu. It's everything you want.

Which parts of spell slots, long rests, Action Surges, and adventuring days are going to be compatible with this idea?

Pulp Cthulhu also explicitly has rules for the 1930s.

Trail of Cthulhu is a game that is filled with research on the 1930s and is much better for the 30s. It’s filled with adventures and ideas that don’t even need eldritch stuff!

Savage Worlds

If I was asked to put this together without a specific system requirement, I would be looking at Hard City (Osprey) as a base and probably Berlin - The Wicked City (Chaosium) for inspiration filling out factions and flavour.

If you don't want a niche book/system, Cthulhu style and Forged in the Dark books will offer very specific materials within a mainstream system actually suited to the game type you are describing.