So you believe it would be okay for Boeing to have a government protected monopoly in this country? You're not worried that Boeing Executive's wouldn't convince key Congress people to make import tariffs higher and higher on Airbus so they could keep jacking up their own prices?
Imposing import tariffs does not create a monopoly for Boeing; it does just the opposite.
With tariffs on imports from unfair competitors, nothing stops Ford or Douglas-Curtis or "CWOJackson Air" or "Cacophonous Airframes" from making airplanes. Preventing unfair competition would ENCOURAGE domestic competition, lowering prices. Boeing could press Congress all they want; if they get undersold by "Two Guys and A Crescent Wrench Manufacturing" in Kiron, Iowa, all the lobbying in the world won't save them.
Then, of course, Airbus would see that is was losing business and start to play fair. Or go out of business because they can't outsell "Redneck Aeroplanes" of Montvale, Virginia. Either way is OK.
What Airbus already has is an effective monopoly because they are protected from going out of business (and hence from bad business practices, etc.).
The simple answer is for the fed to collect a tariff on AirBus sales here in the US and lower Boeing's corporate taxes. Then when Boeing competes with AirBus elsewhere in the world the fed funds the gap to match AirBus from the tariffs they collected on previous US sales. After the sale is won the fed collects a tax on the overall deal as compensation for helping them get the sale they would have otherwise lost, effectively lowering the overall "subsidy". This also encourages them to make a better cheaper product for future deals and not rely on the fed.
Also remember these planes can be in use for 20yrs, think of the profits that are made on parts and repair. The fed could forgo the "deal tax" and collect incrementally over the life of the aircraft instead.
The idea is to allow Boeing the ability to compete dollar for euro and win the contract based on product quailty. The other option is to walk away simply walk away from the sale and let the Europeans pick up the tab.