Large and blocky, the Armstrong Whitworth Whitley was, along with the Wellington and Hampden, one of the RAF's principal bombers when the war began and was notable for being its only heavy bomber until the four-engined Stirling arrived. It participated in the leaflet raids when the war began and then in the night bombing offensive where it remained in service until 1942. Whitleys were the first to drop bombs over German territory on March 1940, the first to attack Italy, and also the first to bomb Berlin on the night of 25/26 August 1940. Nevertheless, the introduction of superior types in the mid-war years relegated it to secondary duties during the rest of the conflict and it ended up performing as transport, glider tug, and anti-submarine warfare with Coastal Command. Among its other notable accomplishments, Whitleys were also the first to be equipped with ASV radar and achieved the first ASV-assisted kill of the war.

The prototype first flew on 4 June 1935 and entered service as the Mk. I. This variant as well as the Mk. II follow-up had been withdrawn as bombers when the war began but they were soon replaced by the Mk. III with power-operated nose turrets and the Mk. IV, the first with Merlin engines which replaced the earlier Amstrong Siddeley Tiger radials. Mk. V eventually became the principal wartime variant and accounted for the grand majority of total Whitley production. The ASW variant was designated Mk. VII.