The United States had many different bombers during WW2. I understand that the B29 was only developed towards the end of the war, but in which situations did they decide to use which (B17, B24, etc...)
I understand that each had different roles, but what were they? Thanks
Are you asking within the context of the B-17, B-24, and later B-29? Or are you asking about the roles of all the bombers in the United States Army Air Force and the US navy?
With regards to the difference between the B-17 and the B-24, both were four engine, large payload, long distance bombers. The B-24 being made by Consolidated Aircraft and in service 3 years after the Boeing designed B-17. Between the two aircraft, the B-24 was the later, faster and farther flying heavy bomber compared to the B-17.
With regards to usage, the most common use for both the B-17 and the B-24 tended to be high altitude strategic bombing. Targets like fuel depots, railways, warehouses, production centers were all the primary targets in strategic bombing runs and the B-17 and B-24 performed rather admirably. The key of course being that both planes could operate at very high altitudes and both planes being capable of flying around 2000 miles per mission.
Now between the two aircraft there were some slight operating differences. While both were fielded in the Pacific and European fronts, the B-24 also had the capability to equip depth charges and torpedoes and was used from 1943 onwards as both a strategic bomber and an anti-submarine plane. While the B-17's higher operating ceiling favored it to high altitude strategic bombing, the B-24 inevitably became a more versatile platform, from anti-ship to even being equipped with radar jamming technology and electronic counter measures near the end of the war.
Now that the specifications are out of the way, the decision to use which plane depended on the individuals within the US high command. Some favored a mildly faster bombing platform and utilized the B-24 while some favored the safer and more rugged B-17. Curiously the RAF did not use the B-24 as a strategic bomber, instead preferring to use the B-24 as a converted supply carrier and an anti-submarine plane and later on an anti-radar equipped jammer. Similarly in the USAAF, the B-24 was used in multiple roles like cargo, but soon overtook the B-17 in the Pacific thanks to the higher speed and the ability to carry more payload.