Avro 683 Lancaster Mark I
Originally the Manchester Mark III was designed with 4 RR Merlin engines to replace the 2 powerful, but unreliable RR Vultures. The Lancaster was immediately successful - so much so that only minor changes were made during its production run, and that the original Mark I was itself never really replaced in production.
Originally designed with a belly turret, these were soon removed. A taboo ring was added around the dorsal turret to prevent the gunner inadvertently shooting the twin tails, and in later marques the turrets were sometimes removed entirely.
Several minor variations to the bomb bay were made, to fit the large size Tallboy and Grandslam bombs.
As deployment to the Pacific War loomed, one idea to increase the range of the Lancaster (and also later the Lincoln) was test flown of a large fuselage fuel tank. The idea was shelved as being too inherently dangerous.
With the end of the European War, many Lancasters were modified to FE (Far East) standard and painted bright white to enable the crews to cope with the heat not built into the European design.
After the War, some Mark I's were modified for photo reconnaissance duties, and were responsible for most of the post war aerial mapping of Africa and the Middle East.
Avro 683 Lancaster Mark II
To cope with a potential lack of Merlin engines, the Mark II was designed around the Bristol Hercules engine. The belly turrets were again built into most Mark II's.
The Merlin engine crisis never eventuated and the Hercules did not live up to their promise, so the Mark II was not produced in great quantity.
Avro 683 Lancaster Mark III
The Lancaster Mark III was indistinguishable from the Mark I, the only difference being the fitting of American Packard Merlin engines - some aircraft were alternated between being a Mark I or Mark III as their engines were changed during service.
The most notable Mark III variant were the Dambuster aircraft, coded for deception purposes as the Vickers Type 464 (provisioning).
After the war some Mark III's were repurposed as Air Sea Rescue aircraft, able to carry and drop lifeboats, and the final Lancaster in RAF service was operated on maritime reconnaissance until 1956.
The Lancaster Mark IV
and Lancaster Mark V
were significant upgrades to the Lancaster design, and became the Lincoln bomber.