Introduction of Merlin XX engines and a twin dorsal turret instead of waist guns resulted in the Halifax B Mk II Series I. The Mk II Series I (Special) achieved improved performance via the removal of the nose and dorsal turrets. The Halifax Mk II Series IA was fitted with a moulded Perspex nose (this nose became standard upon future Halifax variants), a four-gun Boulton Paul Type A dorsal turret similar to that used in the Boulton Paul Defiant, and Merlin 22 engines. The rudder overbalance / directional instability with engine(s) out problem was solved on the Mk III with the fitting of a larger D type fin (40% bigger) and modified rudder. The Mk III Halifax had satisfactory stability in all axes, and in fact were more stable in a dive than a Lancaster, a Lancaster tended to go deeper into a dive whereas a Halifax had to be forced to stay in the dive as the speed increased, i.e. it naturally flew out of a dive.