In the early days of commercial air travel, planes were limited to just 4267m in order to protect their passengers from the the dangers of thin, high-altitude flying. But in 1938, Boeing changed the face of commercial air travel with the model 307 Stratoliner, the first commercial aeroplane to offer a pressurised cabin.
Boeing based the Stratoliner off of its earlier B-17 heavy bomber design from 1935. In fact, the Stratoliner's wings, tail, rudder, landing gear and a whole host of other components came directly from the B-17 production line and were tacked onto a circular, pressurised, fuselage 4m in diameter. What's more, the Stratoliner also borrowed the B-17's four-engine design -- another first for domestic air travel. With its powerful engines and sealed cabin, the Stratoliner was able to climb to more than 6000m into the air, thereby flying above low-lying weather patterns and storms. On board, the Stratoliner's five-man crew and 33 passengers enjoyed a relatively dense atmosphere equivalent to 2400m.
At the time, this was about as baller as air travel could get. Rock star aviationist Howard Hughes thought so highly of the design that he bought one for himself and converted it into a flying penthouse. Only 10 Stratoliners were built in all before the start of the Second World War -- five of which were drafted into the Allied effort and converted into C-75 troop transports -- and today, only one remains: the Flying Cloud. It currently resides in the Steven F. Udvar-Hazy Center near Washington DC's Dulles International Airport on permanent display. [Boeing - Wiki]