I am surprised that pilots on both sides in WW1 got to 20,000 feet for periods of 10-15 minutes without oxygen. They usually got quite ill at the end of it and some blacked out, to recover in a spin at lower altitudes. By mid 1918 they were using oxygen and patrolling up to 21,000 feet. At the extreme altitudes, a height advantage of 50 feet would determine whether you could bring your guns to bear on the enemy, or stall and spin as you raised the nose.