Complex sentence: Proposals for such an aircraft were submitted by Boeing and North American Aviation, and although the program was put on indefinite hold before any actual designs were completed, it paved the way for the B-70 program.
Split into simpler sentences: Proposals for such an aircraft were submitted by Boeing and North American Aviation. Although the program was put on indefinite hold before any actual designs were completed, it paved the way for the B-70 program.