Aircraft that never lived up to their promise

I'll start that one off with the P-39 Aircobra. Any more?

Arthur Kramer

For WW2 era you can't ignore the Brewster F2A Buffalo which was a great
handling machine in its earliest versions but was too heavy and underpowered to
face the Japanese with any significant success. Though like the P-39 in the
USSR the Finnish Air Force did wonders with the Brewster when facing aircraft
that were contemporaries in design era.

also the Heinkel 177 Grief.

John Dupre'