Complex sentence: The Whitley first entered service with No.10 Squadron in March 1937, replacing Handley Page Heyford biplanes, and by the outbreak of the Second World War, seven squadrons were operational with the Whitley.
Split into simpler sentences: The Whitley first entered service with No.10 Squadron in March 1937, replacing Handley Page Heyford biplanes. By the outbreak of the Second World War, seven squadrons were operational with the Whitley.