From Wiki;
307 had GR-1820-G102's fitted with single speed s/chargers,
307B had GR-1820-G105's fitted with twin speed s/chargers.
TWA's 307 were sold to the Army & returned to TWA in 1944, when they were rebuilt by Boeing with wings & tail from the B-17G & refitted with GR-1820-G606 engines & designated 307B-1 - these were later sold to France.