Irish lessor Avolon has signed a memorandum of understanding for 15 Airbus A330-900neos.
The agreement has a list-price value of $4.1 billion.
The aircraft are scheduled to be delivered from 2018. Rolls-Rolls Trent 7000 engines will power the A330neos.
Avolon made the commitment at the Farnborough air show a day after it signed a tentative order for six Boeing 787-9s.
The lessor's chief executive Domhnal Slattery says there is "no doubt" that the sweet spot for the A330neo will be in Asia, where he sees a huge potential in emerging city pairs.
"The A330 is the workhorse and airplane choice in the region," he says.
"This A330neo order complements our ongoing sale-and-leaseback and fleet-financing activity in both the A330 and A320 families, and further strengthens our product offering."
Avolon's owned, managed and committed fleet now stands at 222 aircraft.