Stilton
I have read previously (unfortunately information sources not to hand) that Leigh-Mallory was very much against using fighters as long range escorts. Something like he considered them (primarily?) as defensive tools.
The P-51 was (initially) only reluctantly accepted by the US. Many thousands of P-39s / P-40s etc, were on order & they didn’t want to disrupt deliveries of proven aircraft.
From memory, even in late 1943/early 1944, P-51B/Cs were still being assigned to the RAF & the 9th(?) AF in the intruder role. It was only when Anderson & Doolittle hoovered up all available P-51B/Cs in early February 1944 the 8th AF finally had the escort fighter they needed (& tactics & sufficient drop tanks & well trained pilots). The ‘Big Week’ was one result.
As an aside, it was ‘proven’ in tests by Wright Field that the paper-mâché tanks wouldn’t work ...