Central European budget carrier Wizz Air has tentatively agreed to take 110 Airbus A321neos in a high-density, 239-seat "Cabin Flex" configuration.
Airbus says the aircraft, fitted with 18in seats, would be delivered between 2019 and 2024.
The memorandum of understanding also covers 90 purchase rights.
Flightglobal's Ascend Fleets database shows that Wizz operates 61 A320ceos and has another 23, plus 27 A321ceos, on order.
The airline's A321neo deal is the biggest single order for the variant and pushes A320neo-family orders and commitments beyond 4,000, Airbus indicates.
Wizz operates some 380 routes from 22 bases.