On 9 March 1937, the Whitley Mk I began entering squadron service with No. 10 Squadron of the RAF, replacing their Handley Page Heyford biplanes. In January 1938, the Whitley Mk II first entered squadron service with No. 58 Squadron and in August 1938, the Whitley Mk III first entered service with No. 51 Squadron. In May 1939, the Whitley Mk IV first entered service with No. 10 Squadron and in August 1939, the Whitley Mk IVA first entered service with No. 78 Squadron. By the outbreak of the Second World War, a total of seven squadrons were operational, the majority of these flying Whitley III or IV aircraft, while the Whitley V had only just been introduced to service; a total of 196 Whitleys were on charge with the RAF.