Premise: The Yokosuka B4Y, (Navy Type 96 Carrier Attack Bomber), carrier torpedo bomber was used by the Imperial Japanese Navy Air Service from 1936 to 1943. The B4Y replaced the Mitsubishi B2M2 and was the last biplane bomber used operationally by the Imperial Japanese Navy. The Allied reporting name was "Jean".
Hypothesis: Before the implementation of the Yokosuka B4Y, the Imperial Japanese Navy have used biplane bombers.