https://en.wikipedia.org/wiki/Gettysburg_Cyclorama
https://www.nps.gov/gett/learn/historyculture/gettysburg-cyclorama.htm

It was first completed in the 1880s.
It took a year and a half to get done. There were many assistants who worked with the artist on the painting.
It was recently restored and moved to the museum and visitor center in 2008.