Emirates' agreement includes conversion rights to switch the aircraft to 787-9s, offering the airline additional flexibility for its future fleet and global network.
Emirates is the world´s largest Boeing 777 operator and will be the first to receive the new 777X in 2020.
The deal has been blocked by demands from Emirates - the biggest customer for the twin-deck jet, with 100 of them in service and a further 42 on order - that the pan-European plane-maker continues making the A380 for at least another decade.
H.H. Sheikh Ahmed bin Saeed Al Maktoum, chairman, Emirates, commented, "Emirates' orders today will be delivered from 2022, taking the airline well into the 2030s".
"We are excited that Emirates has selected the Boeing 787-10 Dreamliner to power its fleet expansion and future growth", Boeing Commercial Airplanes CEO Kevin McAllister said in a statement. "Emirates' endorsement of the 787 Dreamliner extends our long-standing partnership and will sustain many jobs in the United States".