Yes, they did. They have several historians on staff that do research for the overall project, as well as the writers themselves doing research - I know for AC2, Corey May spent at least nine months just doing research on Renaissance Italy before he even wrote a single word.
I watched videos on how they made some of the stuff, and the most intriguing bit I watched was that they acquired old maps of building layouts of Boston and New York from that time period, and made the ingame cities as they were back in that time. They said if you had gone through the city back then as you did in game, it would look very much like in game.
I never learned about American History, and I found the events of this game to be pretty interesting. I'm not sure how accurate the events and details are though, but I have yet to see someone complain how about how inaccurate the game actually is so I'll take the game's word for it lol.
Yeah, History in my school doesn't sound that great. At the minute I'm learning about the Partition of Ireland which spans the period 1900-1925 and before that module I did the Clash of Ideologies which pretty much went through the rise to the fall of Communism in Russia.
I never learnt about American History where I live so it was quite interesting to play this game. Quite a few games these days that are set in the past do as much planning as the AC team so it's great for us players to enjoy them and learn something in the process.