In this case, bomber Il-4 with Berezin UB is from 26 March 1942, torpedo bomber Il-4T with Shkas and Shvak - 1 April 1940. I am not sure where to draw the line between DB-3 and Il-4, seems naming change was applied even to some of the already manufactured DB-3
Berezin UB was developed in early 1941 but Il-4 is listed as having Berezin UB in 1939. Precursor of Berezin UB? Actually, Il-4 was produced during 1942-1944. It's precursor would be DB-3, which uses 20mm ShVAK cannon and ShKAS machine guns. DB-3 was produced during 1936–1939.
"Berezin UB was developed in early 1941 but Il-4 is listed as having Berezin UB in 1939. Precursor of Berezin UB?" Actually, Il-4 was produced during 1942-1944. It's precursor would be DB-3, which uses 20mm ShVAK cannon and ShKAS machine guns. DB-3 was produced during 1936–1939.