The role of altitude and oxygen deprivation in flight was known prior to powered flight. Humans had long experienced altitude sickness by ascending mountains. And balloonists had reached thin air long before winged craft. In 1862 Maxwell and Glaisher had accidentally ascended (without oxygen) by balloon to nearly 40,000 feet and only survived by miraculous good fortune and endurance. In 1901 Berson and Süring had reached 35,000 feet by balloon using bottled oxygen. Winged aircraft had reached “dangerously thin” air (15,000+ feet) prior to WW1. By the 1920’s commercial passenger aircraft, flying at 10 to 20 thousand feet, provided bottled oxygen to passengers “in distress”. In 1938, Boeing launched the first pressurized commercial passenger aircraft, the Boeing 307. By the time of WW2, the role of bottled oxygen in high altitude flight was universally known and understood. Virtually all combat aircraft flew with bottled oxygen, usually in integrated systems.

Just to touch on the temperature side of things and resulting clothing requirements, that too was well understood from ballooning days. Richard Crosbie, a pioneering Irish balloonist, reached perhaps 18,000 feet on his first manned flight in 1785 and wore: "... white satin lined with fur, and over all an oiled silk loose coat, lined with the same, red leather boots, and a superb cap" leading Faulkner's Dublin Journal to "... pronounce him one of the finest figures we have ever seen."

For heavier-than-air flight early flying clothing was derived from motoring clothes, a stout tweed suit being sufficient for the low speeds and altitude of the very earliest aircraft, but as aircraft performance improved so more dedicated clothing was introduced. An article from Flight magazine of November 19th 1915 offers to assist aviators who need a suitable outfit:

"Withal, you require winter clothing, Could it be spread out in splendid profusion at your aerodrome, and you had nothing to do but choose and pay, you would refit in no hesitating manner. But to make the round of London's show-rooms, to walk up to an immaculate being of your own sex (generally in the wrong department) arrayed in faultless morning attire, and explain that you require a leather overcoat lined with sheep-skin woolly side out, arouses in you a feeling of humbleness which his 'Step this way, sir' cannot entirely remove. And in your agitation, being a man, you are pleased to grab the nearest available article to your requirements, and—wish with me that clothing could still be shot or speared."

The author tours various London stores listing the aviation clothing on offer; at Harrod's, for example:

"Here are some of the lines I recommend for inspection. Long coats, short coats, three-quarter coats—how shall I ever describe them? Sheepskin jackets, lambswool waistcoats, leather suits, overalls, underalls, gloves, boots, caps—oh dear! A beautiful aviator's three-quarter coat (the adjective governs the coat) in black or tan sheepskin, chrome dressed, and really soft. All-round belt, large side pockets and diagonal map pocket; double breasted, with extra wide overlap fronts, and double collar to turn up, with cross piece to button over the throat. Lined with dark grey fleece, it makes a really serviceable coat. The same in half length, at less money but no reduction in quality. Complete leather trouser suits in black or tan, and a Naval coat, also in both colours, with the usual side pockets and large map pocket. Gloves of every description, lined with every conceivable material calculated to keep the hands warm. One, in particular, I noticed, with the underside of the finger tips left unlined in order to facilitate the feeling of switches. Another good line is a gauntlet glove, lined with fleece, with waterproof canvas backs and leather fronts, and strap fastening. Flying caps of every description, lined with fur, or fleece, or unlined. Helmets, wool under-caps. Long boots, short boots, boots lined and boots unlined; everything one could possibly want in the way of equipment."

At the time there was little in the way of specialised flying clothing in the armed forces so both civilian and military pilots would have been browsing at Harrod's. In 1916 Sidney Cotton of the Royal Naval Air Service was working on his aircraft when there was a sudden scramble, and after landing he noticed his oil-soaked overalls had kept him warmer than the rest of his squadron. Developing the idea he had a one-piece flying suit made with layers of fur and airproof silk under a light Burberry material, a design that proved highly successful and became standard military issue towards the end of the war as the Sidcot suit. For some idea of the complete outfit an airman might wear, an Observer of IX Squadron in the winter of 1918 dressed in:

"... heavy woollen underwear, silk socks underneath wool socks, slippers (I think these were of the ladies bedroom variety) with leather soles lined with felt and woollen uppers, overshoes, silk gloves under combination leather gloves with adjustable fur-lined mitt to cover fingers, Sidcot suit, woollen balaclava helmet under my fur-lined helmet, and face mask and goggles, both lined with fur."

There's another thorough description in Denis Winter's The First of the Few:

"Silk underwear, close-woven woollen underwear duplicating the silk and worn loose, cellular two-inch squared vest, silk inner shirt, Army khaki shirt, two pullovers, tight woven gabardine Sidcot Suit lined with lamb’s wool and muskrat-lined gauntlets with silk inners. Thus dressed the pilot could tolerate temperatures of minus fifty degrees Centigrade – though high wind, poor fit, sweat before dressing or the poor circulation of an unfit man could chill him to the point of tears at ten degrees Fahrenheit. [...] The final adjustment would be to the head area. A silk scarf would be wound carefully around the throat to prevent air entering the vulnerable neck area and getting inside the flying suit as well as preventing skin chafing from that constant turning round in flight to check for enemy behind the tail-plane. The face would then be smeared with whale-oil, surrounded by a balaclava helmet and covered with a non-absorbent face mask, ideally of Nuchwang dog-skin from China. If dog-skin was unavailable, the mask would be wolverine fur, favoured anyway by the Canadian flyers since breath would not freeze on it. The triplex goggles, which covered the single gap in the mask (32s 6d over a London shop counter) were of fur lined moulded sponge-rubber with sage-green-tinted-glasses to absorb ultra-violet rays. Various preparations would finally be rubbed on to the glass to counter fogging below ten degrees Fahrenheit and frosting at minus ten, with perhaps a touch of ointment on the lips, though pilots philosophically accepted the facts that all lips cracked at altitude whatever specifics were used."

As operational altitudes increased and temperatures consequently decreased the RFC and RNAS introduced electrically heated under-jackets, gloves and shoe inserts, such as this example in the Imperial War Museum. Power was delivered by a small windmill generator mounted on the aircraft, though if the generator had no regulator then diving at high speed could result in an unpleasantly high temperature from the increase in voltage delivered.

Though enclosed cockpits gave some protection from the elements, unpressurised aircraft were still subject to extremely low temperatures at high altitude so the flying clothing of the Second World War remained broadly similar, the RAF continuing to use versions of the Sidcot suit with electrically heated clothing for particularly cold conditions until the post-war introduction of pressurised cabins as standard.

See also: "A Brief History of Flying Clothing", Dr Graham Rood, Journal of Aeronautical History Paper No. 2014/01