Complex sentence: Work on using B-17s to carry airborne lifeboats had begun in 1943, but they entered service in the European theater only in February 1945, also being used to provide search and rescue support for B-29 raids against Japan.
Split into simpler sentences: Work on using B-17s to carry airborne lifeboats had begun in 1943, but they entered service in the European theater only in February 1945. They were also used to provide search and rescue support for B-29 raids against Japan.