Emirates' tentative agreement for 40 Boeing 787-10s will, once firmed, take orders for the new twinjet to more than 200.
Boeing had secured orders for 171 of the variant, the largest version of the 787, by the end of October.
These included 163 distributed between nine identified operators – among them Emirates neighbour Etihad Airways – plus eight for an undisclosed customer.
Emirates is expecting to take delivery of the 787-10s from 2022, said chairman Sheikh Ahmed bin Saeed Al Maktoum during the Dubai air show.
The carrier had been weighing the 787-10 against the Airbus A350-900.
Al Maktoum described the 787-10 as "the best option", stressing that Emirates' analysis did not focus simply on price.
Boeing says the agreement includes conversion rights to swap to the smaller 787-9. The -10s will be delivered with both two- and three-class cabins, capable of accommodating 240-300 passengers.
FlightGlobal understands from a source familiar with the discussions that no Airbus A350 agreement is in play for Emirates during the Dubai show – although this does not necessarily rule the twinjet out of the airline's plans.
Emirates had previously been a customer for the A350 with orders for 70 – a mix of -900s and -1000s – but it scrapped the deal in mid-2014.
Although the 787-10 agreement amounts to barely half the number of cancelled airframes, Al Maktoum underlines that the carrier still has dozens of Boeing 777X and Airbus A380 jets on order.