Norwegian has 100 737 MAX 8s on order and options on a further 100, eight of which are now firmed.
It has also signed an agreement with CIT Aerospace to lease two 787-9s, which will be delivered in spring 2018, taking it to a total of 42 787s by 2020.
The Oslo-based carrier has been incrementally increasing its leased 787 commitments over the last few months. It currently operates eight 787-8s and two 787-9s.
"In order to offer customers more routes and make our intercontinental operation even more competitive, we need more brand-new, cost-efficient aircraft. Our intercontinental operation is crucial to the company's global growth and the creation of new jobs," Norwegian CEO Bjørn Kjos said.
In March 2016, Kjos said he was considering placing a follow-on order for more 787-9s in "a couple of years." Any follow-on order would be for 787-9s, rather than the -8 variant, although Kjos has no plans to dispose of Norwegian's eight -8s.
Norwegian would ideally like to operate 50 787s from London Gatwick alone, but the airport is capacity constrained, limiting the airline's ambitions.
The US Department of Transportation (DOT) recently gave tentative permission for Norwegian's Irish subsidiary Norwegian Air International (NAI) to serve the U.S., more than two years after NAI applied for a foreign air carrier permit. When the permit is finalized, NAI plans to begin operations to the U.S., including the first-ever service between Cork, Ireland, and Boston.