To answer comments on my questions:
My thinking is to initially provide a broadbrush approach, with just the minimum to get the answer. In this instance, I deliberately said radical layout, but added the rider of 50 years, rivals, and the undercarriage. Now, if someone could have found a different aircraft that met all those criteria, naturally, I would have agreed, and passed it over. However, I wanted it to be a challenge, and, personally, I can't think of any aircraft that met all those criteria, except the Avro Manchester (actually, that's the correct answer, but I accepted Lancaster, as it's part of the same family).
Was it too hard? I like to slowly add clues, to gradually make it more obvious.
For those wondering about the question, and answer, it's like this:
Roy Chadwick in England, and Ernst Heinkel in Germany, both had remits for heavy long distance bombers (the Germans also wanted theirs to dive bomb, but that's another story). Both designers had access to new engines which were coupled, in England the Vulture, which was 2 Merlins driving a single shaft, whilst the German engine was composed of two DB600 engines. The idea was to reduce drag, etc, by having a single engine housing, and propeller, but in that housing, a twin engine. Both aircraft had problems with their coupled engines, especially over-heating.
Roy Chadwick re-designed the Manchester, as it was now known, in to the Lancaster, with 4 Merlines. The Germans meanwhile, refused to give up on a good idea. The He-177 'Greif' was, in the end, a failure, with few successes, and disliked by it's crews, as well as losing many to mechanical failure. I believe that the Lancaster could carry more than any other bomber in WWII, most famously, the Upkeep and Grand Slam bombs.
In 1944, the Lincoln bomber was produced, as an updated version of the Lancaster, seeing active service (although I believe not used operationally) in WWII. Other versions of the Lancaster were: the Lancastrian civil passenger version of the Lancaster; the York, which had a redesigned fuselage; the Tudor, a much altered passenger version of the Lincoln; and the Ashton, a jet powered airliner version of the Tudor. Not bad for a bomber of WWII!
However, in 1949 came the Shackleton, which would last until 1991. It was basically a Lincoln with a new fuselage section, but retaining the mainplanes, undercarriage, etc. The Shackleton was also a bomber, initially, but I don't believe it was ever used as one, although it retained the capabilies of one until the end. The Mk.III had the tricycle undercarriage.
Thus, from the Avro Manchester, which was a lacklustre aircraft, due to a radical attempt to reduce drag, one has a family of aircraft, from long-range sea patrol to jet airliners, to heavy bombers. A truely remarkable acheivement of Roy Chadwick, who was sadly killed in an Avro Tudor.