So, a suggested Japanese bomber line, Tiers 3-8
Tier 3
Mitsubishi G3M https://en.m.wikipedia.org/wiki/Mitsubishi_G3M
Tier 4
Mitsubishi Ki-21 https://en.m.wikipedia.org/wiki/Mitsubishi_Ki-21
Tier 5.
Mitsubishi G4M https://en.m.wikipedia.org/wiki/Mitsubishi_G4M
Tier 6.
Yokosuka P1Y https://en.m.wikipedia.org/wiki/Yokosuka_P1Y
Tier 7.
Nakajima G8N https://en.m.wikipedia.org/wiki/Nakajima_G8N
all of these actually flew, although the last one only had 4 built. It’s a solid list with a natural progression .