It’s longevity as an effective aircraft?
The Vickers Wellington was a 1932 design to achieve a bomber that met the Geneva Conference maximum weight of 6,000 lbs.
This was one of those daft criteria that the disarmament conferences of the 30’s kept on coming up with.
But Vickers reckoned that Barnes Wallis’s geodetic construction would allow them to meet the maximum weight and still have a superb bomber.
Like every other mid-30’s bomber it only had two engines. But, it had a superb range and payload carrying capacity – at the expense of performance.
It was also incredibly tough and crews flew back in bombers that were heavily damaged.
So, once bombing Germany got too risky the Wellington was shifted to other theatres of operation – like the Mediterranean and Coastal Command where it’s lack of performance was not a serious handicap and it’s range and load carrying ability was very useful.
It was up-engined with Bristol Hercules engines of double the horsepower of it’s original Mercuries and remained a very useful weapon.
So, it was in production for a very long time and was still very useful till the end of the war.