It's a Brechtian epic drama relating the events leading up to dropping the atomic bomb on Japan. Most of the characters are real-life historical people: President Truman, Enrico Fermi, Robert Oppenheimer, Stalin, Churchill.
Here's the author's note to the script:
Enola Gay is a history play. The events in it have been authenticated by multiple sources, and in many instances are part of the historical record, whether it be from the published or unpublished letters, diaries and memoirs of the key participants in the story, [or] official transcripts of the meetings. Scenes of direct address by specific characters are verbatim.
These are the characters own words. In the private meetings of the characters when the doors are closed and only brief quotes are available of what was actually said, I have used my imagination. Of course, this does not make it a documentary, but for such a topic, there is no loose playing with the facts. The] dates of many scenes are part of the historical record.