The Seattle Times (if you want to believe them) has a good read with background information about how the deal came along:
Business & Technology | Boeing scrambles to avoid losing American Airlines deal to Airbus | Seattle Times Newspaper
Apart from the business side, I wonder how AA will manage their new fleet structure, both with respect to routes and crews. Quite an amount of training, simulator rides and type ratings necessary to make it all work, not to speak about maintenance, spares and cabin crews.