The first Lancaster prototype (which was initially called an Avro Manchester Mk III) was BT308. It was basically a Manchester but with bigger wings to incorporate the 4 RR Merlin X engines. It still retained the central tail fin the Manchesters had. It first flew on the 9th January 1941. 

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The 2nd Lancaster prototype, the first to have the now iconic twin tail instead of the triple tail of the Manchester was DG595. This was the first "Lanc" to be fitted with the RR Merlin XX engines that would be standard on the MK I's. It was also the first to be fitted with gun turrets. These two aircraft were the only 2 Mk I prototypes built before full production was ordered.

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The 3rd prototype was DT810. This was fitted with Bristol Hercules VI radial engines due to the shortage of RR Merlin engines at the time. This configuration became the Avro Lancaster II. A second Mk II aircraft (DT812) was ordered, but cancelled before it was built.