Avro Lancaster prototypes

Since the Avro Manchester came out, Roy Chadwick, the designer of the Lancaster, had been working on an improved design that included reforming it so it would be powered by the more reliable Rolls-Royce Merlin engines that were developed for the earlier Bristol Beaufighter Mk.II. Though initially the improved version was going to be named as Manchester Mk.III, it was decided to rename it as Lancaster.
The first prototype, with the registration BT-308, was assembled at Ringway airport, in Manchester, from a Manchester airframe with a modified central section to host the additional engines. On 9th January 1941 it made its maiden flight piloted by test pilot H.A. "Bill" Thorn, at RAF Ringway, in Cheshire.
The test proved to be highly successful than the Manchester and was highly liked. The first prototype had a three-finned tail configuration due to having been adapted from the Manchester. It was quickly revised for the second prototype, with the registration DG-595, and was replaced by the iconic elliptical twin-finned tail that had also been fitted in some late production Manchesters, discarding definitely the central third tail-fin. That removal provided greater stability and a greater field of view and fire to the dorsal gunner. The second prototype was also powered by four Rolls-Royce Merlin XX engines.
After the satisfactory results of the prototype flights, most of the Manchester that were already being manufactured were converted into Lancasters as they shared many common features such as the greenhouse cockpit, turret nose and, on later models of the Manchester, twin tail. The designs were so similar that the entire batch could be completed as the Lancaster Mk.I instead. In fact, the performance was so good that RAF's high command took the decision of reequiping twin-engined squadrons with the Lancasters as quick as possible. The first production Lancaster Mk.I, registered as L7527, flew for the first time in October 1941.

Sources:
1. https://en.wikipedia.org/wiki/Avro_Lancaster
2. Signal Squadron - Aircraft In action 52 - Lancaster in Action