Delta Air Lines has ordered 37 more Airbus A321s, disclosing the deal a day after it committed to 125 Bombardier CS100s.
The US carrier has two A321s in its fleet and 43 on order from previous agreements. Atlanta-based Delta says the 80 on-order aircraft will be delivered through 2019.
Incoming chief executive Ed Bastian says in a statement that "the order for the A321s is an opportunistic fleet move that enables us to produce strong returns and cost-effectively accelerate the retirement of Delta's 116 MD-88s in a capital-efficient manner".
The A321s will be powered by CFM International CFM56 engines.
On 28 April, Delta announced a firm order for 75 CS100s and options on another 50 of the clean-sheet Bombardier narrowbody.
It has now placed its third order for A321s, having committed to 30 in September 2013 and another 15 in June 2014.
Delta will introduce its first A321 on flights between Atlanta and Orlando on 2 May.