Question:  As of September 2017, the Delta Air Lines mainline fleet comprises 852 aircraft in service, merging with Northwest Airlines in 2009, Delta Air Lines operated an all-Boeing and McDonnell Douglas fleet; it incorporated into which company's Airbus aircraft into its fleet following the merger?
Sentence (from "Delta Air Lines fleet"): As such, it has one of the oldest fleets of any American airline, with an average fleet age of 17 years as of 2017.
Relevant evidence: No