Recasts with Trump quote in paragraphs 4-5, criticism from Democrats in paragraphs 9-10

By Doina Chiacu and Steve Holland

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 BA.N 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 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.

(Reporting by Doina Chiacu and Steve Holland; Additional reporting by Richard Cowan, Andrea Shalal and Jared Renshaw; Editing by Colleen Jenkins and Nia Williams)

((doina.chiacu@thomsonreuters.com; 202-898-8322;))