Hi Frank3,
I guess it's a Martin B-10, though I only know a picture of a B-10B with nose gun position, enclosed cockpit and a closed canopy for the crew member in the rear fuselage.
In 1934, 10 B-10 bombers made a non-stop flight from Alaska to Seattle to prove a point about long-range capabilities.
In the Spanish Civil War, the Tupolev SB was called "Martin bomber" by the Luftwaffe, who apparently didn't believe the Soviet Union could produce a sophisticated bomber. (The SB was ahead of the B-10, actually.)
Regards,
Henning (HoHun)