When composing a list of the country’s buildings that are most worth saving, the hangar for the Enola Gay at Wendover Airfield in Utah might not come immediately to mind.But when the National Trust for Historic Preservation assembles its annual roster of America’s most endangered historic places, it looks for more than aesthetic distinction. Each year the trust selects what it considers important examples of the nation’s architectural, cultural and natural heritage that are at risk of being destroyed or irreparably damaged.So when the trust unveils its 2009 sites on Tuesday, the hangar will be among them. It housed the Enola Gay, the plane that dropped the world’s first atomic bomb used in war, on Hiroshima, Japan, in 1945, and is in critical disrepair. Other similarly less-than-glamorous locations on the list are Memorial Bridge, which for more than 85 years has connected the coastal towns of Portsmouth, N.H., and Kittery, Maine, and is now in danger of removal, and the Human Services Center in Yankton, S.D. Founded in 1879 as the South Dakota Hospital for the Insane, the institution’s collection of neo-Classical, Art Deco and Italianate buildings have long stood vacant, and the state plans to tear down 11 of them.“Buildings like that can be adaptively reused for new community purposes,” said Richard Moe, president of the National Trust for Historic Preservation. “It’s a mistake to allow structures to fall into disrepair or to be demolished.”The current economic downturn is a mixed blessing for endangered buildings, Mr. Moe said. Although more buildings are being neglected, fewer are threatened with demolition because development has slowed. In 22 years the trust has selected 211 sites worth saving and lost just 6 of them.