You're comparing apples and oranges.

The Mosquito and the B-25 Mitchell (and the B-26 Marauder, for that matter) were built with different doctrine and tactics in mind. The Mosquito was designed to operate semi-independently and use speed as its defense. The B-25 and B-26 were medium bombers designed to bomb in tight formation from medium or high altitudes of ~10-25,000 feet. Altitude and aerial gunnery were meant to be their defense.

By 1942 North American was already rolling B-25s off the line and Martin was building B-26s. Suddenly stopping production right as the USAAF desperately needed medium bombers would have been a terrible strategic decision. Retooling and overhauling factories to build an brand-new, all-wood aircraft would have meant months of delays and a huge aircraft production shortfall at the worst possible moment.

The USAAF did make some use of Mosquitos they got from the British, primarily in the reconnaissance role.

A better comparison might be between the A-20 Havoc and the Mosquito. The Havoc was built as a light bomber, meant to attack at low- and medium-level. However, ending production of the metal Havoc to build wooden Mosquitos would also have meant massive disruption to production. Shortly before the US entered the war, Douglas had build a massive factory in Santa Monica to build the Havoc. They would have had to throw away millions of dollars in brand-new tooling, retrain thousands of workers, find new suppliers and contractors, and learn an unfamiliar aircraft construction technique. For obvious reasons, this was a bad idea.

When it came to buying aircraft, the RAF bought (or was essentially given through Lend-Lease) large numbers of US aircraft. Indeed the RAF would go on to use Douglas Bombers (they called the A-20 the "Boston") with great success. The RAF had a massive shortage of light bombers and the Boston fit the bill quite nicely!

Hey there,

Just to let you know, your question is fine, and we're letting it stand. However, you should be aware that questions framed as 'Why didn't X do Y' relatively often don't get an answer that meets our standards (in our experience as moderators). There are a few reasons for this. Firstly, it often can be difficult to prove the counterfactual: historians know much more about what happened than what might have happened. Secondly, 'why didn't X do Y' questions are sometimes phrased in an ahistorical way. It's worth remembering that people in the past couldn't see into the future, and they generally didn't have all the information we now have about their situations; things that look obvious now didn't necessarily look that way at the time.

If you end up not getting a response after a day or two, consider asking a new question focusing instead on why what happened did happen (rather than why what didn't happen didn't happen) - this kind of question is more likely to get a response in our experience. Hope this helps!