Boeing Commercial Airplanes President and CEO Ray Conner said having the Japanese make more parts for the 777X was considered, but the decision was to have Boeing make the wing.
"This is a partnership that will endure for many, many years to come," Conner said after Boeing, Mitsubishi Heavy Industries, Kawasaki Heavy Industries, Fuji Heavy Industries, ShinMaywa Industries and NIPPI Corp. signed the deal Thursday at a Tokyo hotel.
Twenty-one percent is the same level Japanese manufacturers got for the predecessor 777 aircraft, one of the most popular commercial jets on the market.
The Japanese manufacturers said the contract was a reflection of their reputation for quality production and punctual deliveries.
The 777X, set for delivery from 2020, is billed as 12 percent more fuel efficient than rival Airbus's A350.
Boeing already has 260 orders for the jets, including an order from Japan's All Nippon Airways.
Japan Airlines, which has had a long relationship with Boeing, has not ordered them yet.
ANA was the first customer for Boeing's 787 and JAL also bought some of the planes.
But last year, JAL spurned Boeing and ordered from Airbus for the first time with a deal for 31 A350 planes worth $9.5 billion at list prices.
Even after delivery, the 787 was plagued with problems including batteries that were prone to fires. Boeing has said it dealt with that problem by encasing the batteries to contain any overheating.
Earlier this week, Emirates Airlines cancelled its order for 70 of Airbus's A350 aircraft, the European maker's answer to the domination of the 777 and 787. The A350 has also been plagued by delays. Emirates has ordered 150 777X jets.
The 777X variants include the 777-8X, whose list price is $349.8 million and carries up to 350 passengers. The list price for the 400-passenger 777-9X is $377.2 million.
The companies declined to disclose monetary terms of Thursday's deal.