To their defence, in other countries also it happened almost by accident.
- in the UK they were essentially forced to adapt the concept after the complete failure of the Fairey Battle and the obsolence of the Hawker Hurricane, which production lines were making thousends of them and they had to find a role or have a serious disruption of fighter production (More by luck than design, the Hurri's fat aerofoil gave a life extension to a dated design, with the Mk.IIc version). The "kittybomber" was yet another adaption of a fighter plane to a fighter bomber for much the same reasons, even if it was more competent as a fighter. Even the Typhoon was not designed with that role in mind, but came late enough to be adapted to this role and avoid cancelation.
- In the US they used the british experience and benefit from the fact they entered late in the war.