Under developed or discarded aircraft

As TAG says, the Bloch was tested extensively for a few months, but never with guns or ammunition fitted - and probably not armour either. This would have meant a much lighter all-up weight for speed testing as compared to in service.

While the Whirlwind was a bit of a dead end...the concept wasn't; I've read it argued that the DH Hornet and Sea Hornet were the real fulfilment of the idea of a heavy but fast single-seat British twin-engined fighter..