If you read the Wikipedia link on the plane, you'll see it was the US Army Air Corps' first-line medium bomber in the late 1930s. Only five are known to exist, none in flyable condition.
From today's perspective, when we're flying 40+ year-old B-52s, it's almost amazing to think that this plane entered service in 1936 and was considered obsolete when the US entered WWII in 1941. It was widely deployed at the beginning of the war, and the majority of the bombers destroyed by Japanese bombing on 7 and 8 December 1941 in Hawaii and the Philippines were B-18s.
The remaining B-18s were mostly reassigned to coastal patrolling and replaced by the B-25s and B-26s that bore the brunt of war service. Coastal patrolling was soon taken over by PBY Catalinas and B-24 variants. B-18s spent most of the war as training aircraft or were converted to transports. During the war, Lowry was a center for bombardier training, and this picture shows B-18s on the flight line there with a group of cadets.
Another very cool exhibit at the museum was this Norden bombsight. This was a technical marvel and closely guarded secret at the beginning of the war. I have been reading about these for ages, and this is the first I have seen. It occurred to me that this was the same type bombsight used by Steve's father who was a bombardier in B-17s over Europe during the war.