The increase in the length of the 777-8's fuselage has boosted the aircraft's passenger capacity and range.
The Gulf carrier has firm orders for 115 Boeing 777X aircraft.
The order was previously listed as belonging to an unidentified customer in Boeing's order books.
More good news for Boeing on the opening day of the Farnborough Airshow.
Ethiopian Airlines is taking the next step in its vision to become a leading global cargo provider.