By KEVIN FREKING, Associated Press

WASHINGTON (AP) — Taking a cue from his wife, President Barack Obama challenged Americans on Thursday to consider how they might give back to those who have served in the military and their families.

Obama spoke to more than 1,300 service members at Joint Base Andrews as part of an anniversary celebration honoring the USO and the "Joining Forces" initiative that first lady Michelle Obama and Jill Biden created in 2011.

"I challenge every American to ask that simple question: How can I give back to these troops and families who have given me so much? Obama said. "Everybody should have that in their minds."

Obama said the celebration, which featured comedians Jon Stewart and David Letterman, represented a marker for Joining Forces rather than the end point even with just eight months left in his administration. He said about 200,000 service members make the transition to civilian life every year, so the need to help them will continue.

About 2.7 million people have returned from serving in war zones since September 11, 2001. The first lady said those returning home have in many ways made her job easy when it comes to trying to enlist help from employers and others.

"When people hear about you, they want to step up," she said. "You inspire people all across this county and that is absolutely true for Jill and me."

The focus of Joining Forces has been primarily on helping veterans and military spouses gain employment. Earlier Thursday, Mrs. Obama said more than 1.2 million veterans and spouses have been hired or trained since the launch.

She also announced that 40 companies have pledged to hire more than 110,000 veterans and spouses over the next five years. Jeff Bezos, founder and CEO of Amazon, appeared with her and said the online retailer will hire 25,000 veterans over the period, nearly one-fourth of the total.

"So we need to keep up this momentum," the first lady said. "And I will say this again and again and again: No matter who is in the White House next, this should absolutely continue to be a national priority with national leadership coming from this building."

Stewart and Letterman came to the event with vastly changed appearances from their days hosting television comedy shows. Each sported a full beard.

Letterman had such lengthy white whiskers that he said someone came up to him to thank him for all he had done, including his poetry.

"Who do you think I am?" Letterman asked.

The response: "Walt Whitman."

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Associated Press writer Darlene Superville contributed to this report.