Norwegian has ordered another two B787-9 Dreamliners for delivery in 2017.
The airline said both aircraft are scheduled to enter service on long-haul routes that summer.
The agreement means that the number of Dreamliners in Norwegian's fleet will be 19 by 2018.
Currently, the carrier has eight B787-8s and 11 B787-9s on order. Four will be delivered next year, five in 2017 and two in 2018.
Norwegian will configure its B787-9 in two classes with 35 seats in premium economy and 309 in economy. It is leasing the new aircraft from its asset company, Arctic Aviation Assets.
CEO Bjorn Kjos said: "Our long-haul routes have been very popular and I'm very satisfied to have secured more Dreamliners. This will enable us to launch even more routes to exiting destinations all over the world."
Norwegian will launch new transatlantic route from London Gatwick to Boston next May (see news, July 9).