WASHINGTON, May 12 (Reuters) - U.S. President Donald

Trump dismissed ethical concerns about his plan to accept an

airplane as a gift from the Qatari royal family, saying on

Monday it would be "stupid" to turn down the generous offer.

The $400 million luxury plane, which would be outfitted

to serve as Air Force One, would be one of the most valuable

gifts ever received by the U.S. government. News of the offer

drew immediate criticism from Democrats and advocates of good

government, who warned it was a conflict of interest that could

influence presidential decisions.

Trump said the Boeing 747-8 airplane would

eventually be donated to his presidential library - a repository

housing research materials from his administration - and that he

had no plans to use it for personal reasons after leaving

office.

"I think it's a great gesture from Qatar. I appreciate

it very much. I would never be one to turn down that kind of an

offer," Trump told reporters at the White House before leaving

for a trip to the Middle East.

"I mean, I could be a stupid person saying, 'No, we don't

want a free, very expensive airplane,'" Trump said.

The Republican president attributed the offer to gratitude

for U.S. help in the defense of countries in the region

including Qatar, Saudi Arabia and United Arab Emirates, all

stops on his itinerary this week.

Trump also said accepting it was a practical decision,

and that he was disappointed that Boeing ( BA ) has taken so long to

deliver the new Air Force One planes he commissioned during his

first term as president.

Critics of the offer said it was unethical and likely

unconstitutional.

Democratic Senators Brian Schatz, Chris Murphy, Cory

Booker and Chris Coons said in a statement that Trump's

acceptance of such a gift would create a clear conflict of

interest, raise serious national security questions and invite

foreign influence.

U.S. Representative Joe Courtney of Connecticut, the

senior Democrat on the House Seapower and Projection Forces

Subcommittee, said it would distract from the Air Force's

efforts to speed up delivery of the actual new Air Force One

fleet.

White House spokeswoman Karoline Leavitt said legal details

surrounding the donation to the Defense Department were still

being worked out and that the Trump administration was not

worried about what the Qataris might ask for in return.