No ruling in the case against Airbus is expected before September, with a verdict in the Boeing case due early next year.
About $16.6bn of the subsidies to Boeing had come through research and development support from the US defence department and Nasa, the space agency, which allowed Boeing to use its technology without having to pay for it.
Boeing had also received $2.2bn in export subsidies, already condemned in another WTO case, and $4bn in tax breaks and other benefits from the states of Washington and Kansas as well as other authorities, the EU said.
In a statement, Washington brushed aside Europe's argument, saying that some of the programmes mentioned were not related to civil aircraft while others were available to a range of companies, including Airbus.
"They are not subsidies. In any event ... they are dwarfed by the magnitude of the subsidies that Airbus has benefited from"
Charlie Miller, Boeing's spokesman.
"They are not subsidies. In any event, even if all of the EC [EU] allegations had merit - which they don't - they are dwarfed by the magnitude of the subsidies that Airbus has benefited from," Charlie Miller, Boeing's spokesman, said.
The row started in late 2004 when Washington, under pressure from Boeing, withdrew from a 1992 pact between the United States and the EU on commercial airliner financing.
At the time Airbus had overtaken Boeing as the world's leading aircraft maker in terms of its order book, but the US firm has since reclaimed the number-one spot.
Both sides have long said that they are ready to negotiate a settlement to legal battles that some industry analysts have said both could end up losing.