Now a group of scholars, writers, activists and others have signed a petition criticizing the exhibit for labeling the Enola Gay as ''the largest and most technologically advanced airplane for its time'' without mentioning that the Boeing B-29 dropped the bomb on Hiroshima. When officials at the Smithsonian Institution unveiled a new home for the World War II bomber the Enola Gay in August, they had hoped to avoid the kind of controversy that had previously plagued efforts to exhibit the airplane that carried the first atomic bomb.