 Due to armament difficulties encountered that left the Wellington with weaker than intended defences, the "Wellington Mk IB" was proposed for trials, but appears to have been unbuilt.Further development of various aspects of the aircraft, such as the hydraulics and electrical systems, along with a revision of the ventral turret gun, led to the "Wellington Mk IC".In January 1938, design work on what would become the "Wellington Mk II" formally commenced.