In the Manual, the different bombs are described as such:
General Purpose: Big booms. Good for for buildings and structures
Armour Piercing: Bombs with smaller explosions and damage, but is less affected by armor.
Semi-Armour Piercing: A midway between the previous two.
Incendiary: Makes fire to keep things burning afterwards, good for factories and oil.
Anti-Personnel: Cluster bombs good against troop concentrations.
Also, choosing larger yield bombs results in less bombs to be dropped.

I want to know what targets get what bombs. It mostly seems obvious by the names by the names (i.e troop concentration, factory, submarine, oil), but I don't know what to do in cases of things like bridges, railroad junctions, and airfields which don't seem to obviously fall into those categories, or what yields to choose (like 12 150lb bombs or two 2000lb bombs).

I'm not an expert on the game but from my experience in flying and bombing stuff:

- Brigdes: 2 2000lb bombs.
- Railroad junctions: 12 150lb bombs.
- Airfields: choose semi-armour piercing for cratering hardened runways but normal 150lb for the grassy ones.

Historically B-17's mostly used the standard 12 150lb bomb payload AFAIK.

General purpose for pretty much everything. I only use incendiary for oil refineries, submarine bases are the only thing i can find to justify a fully armor piercing bomb. The only site i ever used clusters on is troop concentrations.

For common factories, i like to mix semi-piercing and general purpose bombs.

Also, the 1600p semi piercing bomb, even though it might not be practical for the target, fits into the B17 perfectly, leaving alot of room for extra general purpose bombs. Try 1600p AP primary with a 250p GP secondary, you might be suprized at to what the ground crew actually loads. I know i was, and now i choose that for almost every factory bombing.

Have fun!