Complex sentence: These tasks were initially performed with Lockheed P-38 Lightnings and Republic P-47 Thunderbolts through the end of 1943, with both earlier types being steadily replaced with the long-ranged North American P-51 Mustangs as the spring of 1944 wore on.
Split into simpler sentences: These tasks were initially performed with Lockheed P-38 Lightnings and Republic P-47 Thunderbolts through the end of 1943. These were progressively replaced with the long-ranged North American P-51 Mustangs as the spring of 1944 wore on.