The decision means the US can now ask an arbitrator to determine how much it can retaliate against the European bloc for failing to comply — raising a new question about how much Washington may recover from the EU through retaliatory tariffs.
Boeing alleges the EU has doled out more than US$22 billion ($32b) in illegal subsidies to Airbus, saying in a statement that the stage is now set for "the largest-ever WTO authorisation of retaliatory tariffs" as early as next year.
The Chicago-based plane maker didn't immediately indicate how it came up with that estimate.
The Trump Administration appeared ready to call on the arbitrator to step in.
"This report confirms once and for all that the EU has long ignored WTO rules, and even worse, EU aircraft subsidies have cost American aerospace companies tens of billions of dollars in lost revenue," said US Trade Representative Robert Lighthizer.
"Unless the EU finally takes action to stop breaking the rules and harming US interests, the United States will have to move forward with countermeasures on EU products."
The ruling centers on actions taken by the EU generally, as well as four of its member states: Britain, France, Germany and Spain.
Deciding on a case first opened in 2004, the appellate body found Airbus had paid a lower interest rate on financing to develop the A350XWB than the European plane-maker would have gotten in the open market.
It also upheld a ruling by the WTO's compliance panel that the launch aid continued to exist after December 1, 2011 - the day that the EU was supposed to comply under a previous decision.
The EU appears set to argue that the only penalty it should face would be on the difference between the market rate for the financing, and the rate that Airbus actually received from the EU.
Boeing's top executive appeared to praise the ruling, and said it was time for "all parties" to work for "the mutual health of our industry".
"Today's final ruling sends a clear message: Disregard for the rules and illegal subsidies is not tolerated," said Boeing Chairman and CEO Dennis Muilenburg.
"The commercial success of products and services should be driven by their merits and not by market-distorting actions."
European Trade Commissioner Cecilia Malmstroem insisted that the ruling also rejected "the vast majority" of US claims against it, while saying in a statement that the bloc "will now take swift action to ensure it is fully in line with the WTO's final decision in this case."
- AP