The Boeing 737-800 is the most commonly used aircraft by airlines in the United States. As of the end of 2017, American Airlines operated a fleet of 304 aircraft of this type, ahead of Southwest with 181 aircraft, United Airlines (141), Delta Air Lines (77), and Alaska Airlines (61). Source: BriYYZ (flickr user).
Recently, the U.S. Department of Transportation released its annual Schedule B-43 Inventory database, which contains all commercial aircraft in the U.S. inventory as of year-end 2017. Some inaccuracies and inconsistencies in the raw data were identified and fixed by Forecast International by cross-checking with individual airline fleet data.