Currently American Airlines has two issuers of credit cards: Barclays & Citi. Barclays. This stems back from when Barclays issued the US Airways credit cards and American Airlines acquired US Airways in 2013, the deals were then renewed in 2016. Barclays is allowed to advertise inflight/in airports and Citi has the rest of the acquisition channels.
CNBC is now reporting that American Airlines & Citi are in talks for Citi to become the sole issuer of the American Airline cards in a new long term deal. When the US Airways merger was under way American Airlines did offer Citi the ability to become the sole issuer, this would have also allowed Citi ThankYou points to transfer to American Airlines as well but Citi was unhappy with the asking price. It seems if the deal is completed that Citi would also purchase the back book from Barclays.
Source: doctorofcredit.com