Rolls-Royce said on Friday that the order for engines also included the provision of long-term post-delivery services as part of a "TotalCare" support package.
The engines ordered are the Trent XWB engines to power 25 Airbus A350s, and Trent 7000 engines to power 25 Airbus A330neo aircraft. Rolls-Royce is the only supplier of engines for both aircraft.
Delta's order was first reported on Wednesday and follows a closely fought competition with Boeing (BA.N), which had offered its 787 Dreamliner.
Reporting by Sarah Young, Editing by Paul Sandle