The boom you heard was an explosion on Placer Drive off of Rimrock Road across from Rocky Mountain College.

From ktvq:

Billings Fire Department personnel have been dispatched to the area of Placer Drive and Rimrock Road following an explosion. We are getting initial reports of damage in the area. We will update with information as soon as possible.

....We will have more details as they become available.

Police and Fire Department are on the scene putting out 30 foot flames and have asked everybody to stay away.

http://www.ktvq.com/news/explosion-rocks-area-near-rimrock-road-and-13th-street/