The Lexington class battle cruisers started life as a proposal in 1903 for a more effective armoured cruiser design, which by 1908 had become a proper battle cruiser in the wake of the Dreadnought launch and the laying down of the Invincible class. The first draft was for a battle cruiser variant of the Wyoming class, the construction of which was not recommended, due also to very limited funding available pre-1917. The further studies were also a response to the Japanese Tsukuba class of fast armoured cruisers initially, and then the Kongo being launched in Britain in 1912, followed by her three sisters the following years.
In 1916, with the Great War fully raging in Europe, the government passed a five-year Navy law, in effect from 1917, calling for the construction of ten battleships, six battle cruisers and ten destroyers by 1922. Therefore the design of the Lexingtons was changed many times between their 1916 authorization and laid down five years later.
The original design was to be armed with ten 14''/50 guns, displace around 34,000 tons and have a top speed of 35 kn. This last requirement was kept throughout the development as their were supposed to operate closely within the new fleet of destroyers (the future Wickes and Clemson classes) and light and scout cruisers (the Omahas). Due to the very high top speed, on the machinery of the day this would have required the vessels to have no less than seven funnels, probably looking more like a 'funnel farm' akin to HMS Agincourt's 'turret farm', which some people would surely have found hilarious. However, after the battle of Jutland the class had to be redesigned in 1917, in view of the lessons (and the wrong uses) battle cruisers could be put to, and the launch of what was effectively the first fast battleship, HMS Hood, in 1918.
The final form of the Lexington class was to be as follows: eight 16''/50 guns in four twin turrets, fourteen single 6'' guns, four single 3'' AA guns, and eight 21'' submerged torpedo tubes. Speed was a projected 33 knots on 180,000 shp (which was not changed from the previous 35 knot-requirement, however additional weights impaired this original speed), and a mighty 5''-7'' belt armour, which could have probably stopped 8'' rounds on a good day. The protection scheme was totally inadequate if they ever found themselves embroiled with the enemy scouting forces and their own battle cruisers – which they probably would give it was their role.
In the end, none of the ships was completed. The Washington Naval Treaty, signed in 1922, saw the vessels only a year into construction, and so four of them were scrapped. The two most advanced hulls – Lexington and Saratoga – went on to be converted into aircraft carriers as per the new treaty and have a totally different service life that was originally envisaged for them.
The Lexington class was the only class of battle cruisers in the US Navy as per official documents. The later Alaska class owed a lot to their design, however for various reasons they were not true battle cruisers, but large cruisers.