At the start of the war aircrew wore service dress uniform while flying. Over the top of the uniform there were a number of options: simple overalls in black, white or blue cotton (the white overalls were issued to display pilots during the inter-war period so something of a mark of status), Sidcot suits (named after Sidney Cotton, who developed the multi-layer one-piece suit in the First World War), or sheepskin-lined Irvin jackets and trousers (more common for bomber crew, as they were rather bulky in confined fighter cockpits, with electrically heated versions also available). Choice depended on conditions and personal preference, pilots operating at particularly high altitude or bomber crew flying lengthy night-time missions needed much warmer clothing, whereas during the Battle of Britain fighter pilots on standby for extended periods in the warm summer often just flew in the uniform (with the tie usually replaced with a silk scarf - "I remove my tie and open the collar of my shirt (far less sore on the back of the neck when you keep looking round). Now where's my silk scarf? Don't say it's been lost.", Geoff Wellum, First Light). New flying dress was introduced in 1941 to replace the service dress uniform in the cockpit: Suits, Blue/Grey, Aircrew, similar to army battledress but in RAF blue (though 2nd TAF pilots operating from Europe after D-Day had a khaki version, to avoid confusion with grey German uniforms).

A few examples from the Imperial War Museum's collection:

242 Squadron, September 1940: Douglas Bader (front & centre) in a Sidcot suit, other pilots in standard uniform or overalls (both black & white).

Roland 'Bee' Beamont, 1944: Beamont in the post-1940 battledress.

Blenheim crew, 1940: service dress uniform and an Irvin jacket on the right

Lancaster crew, 1944: battledress, skipper wearing an Irvin jacket