While the early-built models of the Halifax were heavily used by Bomber Command and made valuable contributions to operations, the aircraft's performance was considered unsatisfactory for the most part, mainly due to the underpowered Merlin engine, which meant that it could not fly at the higher altitudes needed to avoid enemy fighters, which were becoming increasingly effective throughout 1943. This was answered by the Halifax Mk III, which was powered by Bristol Hercules radial engines in place of the Merlins. Introduced into service in November 1943, the Mk III was first delivered to No. 433 Squadron and No. 466 Squadron. By January 1944, the Hercules-powered Halifax was available in quantity and quickly proved to have superior performance in the face of German fighter defences.