Two basic variants of the Canberra featured a Boulton-Paul designed and constructed ventral pack containing four 20mm Hispano cannon mounted in the rear of the bomb-bay with 525 rounds per gun.
The definitive "interdictor" was the B(I).8 with a new, off-set to port, fighter type windscreen and blown canopy designed at the Acton works of Napier which English-Electric had purchased in December 1942.
An "interim" version was the B(I).6 which retained the original canopy of the earlier Canberras.