Lancaster Mk II in flight
Due to concerns over the availability of Rolls-Royce engines, the Lancaster Mk II was developed and first flew in November 1941. It was equipped with powerful Bristol Hercules radial engines, and a total of 300 Mk IIs were produced, differing from the Mk I by having more of them. However, their speed and altitude performance were always inferior to those of the Mk I, as the extra drag and fuel consumption of the Hercules outweighed their power advantage. Once Merlin supplies were secured, further orders for Mk IIs were cancelled. The engine supply issue was ultimately resolved by establishing Merlin production overseas. In August 1942, the Mk III was introduced, which had American-built Packard Merlins, but was otherwise identical to the Mk I in most respects. In fact, it was not uncommon for Mk Is to be re-engined with Packard engines in service, making them Mk IIIs, and vice versa.