The Ten Year Rule assumed that Britain would face no major war for ten years and officially ended in 1932. The practical brakes on rearmament did not come off until 1937, the year Illustrious was laid down. Her design therefore emerged while the Royal Navy was trying to recover from years of constrained planning. That timing helps explain why carrier and aircraft programmes still carried older assumptions as war approached.
Notable Point · 38:02-38:22
The Ten Year Rule and Illustrious's timing
Illustrious was laid down in 1937, the year the practical restraints left by the Ten Year Rule finally eased.