Question: When did the Essex class fleet carriers become available?
About the article "Battle of Midway", section "Impact".

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The Washington Naval Treaty imposed limits on the maximum size and total tonnage of aircraft carriers for the five main naval powers.
Later treaties largely kept these provisions.
As a result construction between the World Wars had been insufficient to meet operational needs for aircraft carriers as World War II expanded from Europe.
Too few fleet carriers were available to simultaneously transport aircraft to distant bases, support amphibious invasions, offer carrier landing training for replacement pilots, conduct anti-submarine patrols, and provide defensive air cover for deployed battleships and cruisers.
The foregoing mission requirements limited use of fleet carriers′ unique offensive strike capability demonstrated at the Battle of Taranto and the Attack on Pearl Harbor.
Conversion of existing ships (and hulls under construction for other purposes) provided additional aircraft carriers until new construction became available.
Conversions of cruisers and passenger liners with speed similar to fleet carriers were identified by the U.S. as "light aircraft carriers" (hull classification symbol CVL) able to operate at battle fleet speeds.
Slower conversions were classified as "escort carriers" and were considered naval auxiliaries suitable for pilot training and transport of aircraft to distant bases.
The Royal Navy had recognized a need for carriers to defend its trade routes in the 1930s.
While designs had been prepared for "trade protection carriers" and five suitable liners identified for conversion, nothing further was done mostly because there were insufficient aircraft for even the fleet carriers under construction at the time.
However by 1940 the need had become urgent and was converted from the captured German merchant ship MV "Hannover" and commissioned in July 1941.
For defence from German aircraft, convoys were supplied first with Fighter catapult ships and CAM Ships which could carry a single (disposable) fighter.
In the interim, before escort carriers could be supplied, they also brought in Merchant aircraft carriers which could operate four aircraft.
In 1940, Admiral William Halsey recommended construction of naval auxiliaries for pilot training.
In early 1941 the British asked the US to build on their behalf six carriers of an improved "Audacity" design but the US had already begun their own escort carrier.
On 1 February 1941, the United States Chief of Naval Operations gave priority to construction of naval auxiliaries for aircraft transport.
U.S. ships built to meet these needs were initially referred to as auxiliary aircraft escort vessels (AVG) in February 1942 and then auxiliary aircraft carrier (ACV) on 5 August 1942.
The first U.S. example of the type was .
Operation Torch and North Atlantic anti-submarine warfare proved these ships capable aircraft carriers for ship formations moving at the speed of trade or amphibious invasion convoys.
U.S. classification revision to escort aircraft carrier (CVE) on 15 July 1943 reflected upgraded status from auxiliary to combatant.
They were informally known as "Jeep carriers" or "baby flattops".
It was quickly found that the escort carriers had better performance than light carriers, which tended to pitch badly in moderate to high seas.
The "Commencement Bay"-class was designed to incorporate the best features of American CVLs on a more stable hull with a less expensive propulsion system.
Among their crews, CVE was sarcastically said to stand for "Combustible, Vulnerable, and Expendable".
Magazine protection was minimal in comparison to fleet aircraft carriers.
was sunk within minutes by a single torpedo, and exploded from undetermined causes with very heavy loss of life.
Three escort carriers—, and —were destroyed by "kamikaze"s, the largest ships to meet such a fate.
Allied escort carriers were typically around long, not much more than half the length of the almost fleet carriers of the same era, but were less than 1/3 of the weight.
A typical escort carrier displaced about , as compared to almost for a full-size fleet carrier.
The aircraft hangar typically ran only 1/3 of the way under the flight deck and housed a combination of 24-30 fighters and bombers organized into one single "composite squadron".
By comparison, a late "Essex"-class fleet carrier could carry a total of 103 aircraft organized into separate fighter, bomber and torpedo-bomber squadrons.
The island on these ships was small and cramped, and located well forward of the funnels (unlike on a normal-sized carrier where the funnels were integrated into the island).
Although the first escort carriers had only one aircraft elevator, two elevators, one fore and one aft, quickly became standard, so did the one aircraft catapult.
The carriers employed the same system of arresting cables and tailhooks as on the big carriers, and procedures for launch and recovery were the same as well.
The crew size was less than 1/3 of that of a large carrier, but this was still a bigger complement than most naval vessels.
It was large enough to justify the existence of facilities such as a permanent canteen or snack bar, called a gedunk bar, in addition to the mess.
The bar was open for longer hours than the mess and sold several flavors of ice cream, along with cigarettes and other consumables.
There were also several vending machines, which made a "gedunk" sound when operated.
In all, 130 Allied escort carriers were launched or converted during the war.
Of these, six were British conversions of merchant ships: , , , , and .
The remaining escort carriers were U.S.-built.
Like the British, the first U.S. escort carriers were converted merchant vessels (or in the "Sangamon" class, converted military oilers).
The "Bogue" class carriers were based on the hull of the Type C3 cargo ship.
The last 69 escort carriers of the "Casablanca" and "Commencement Bay" classes were purpose-designed and purpose-built carriers drawing on the experience gained with the previous classes. 
The Battle of Midway has often been called "the turning point of the Pacific".
However, the Japanese continued to try to secure more strategic territory in the South Pacific, and the U.S. did not move from a state of naval parity to one of increasing supremacy until after several more months of hard combat.
Thus, although Midway was the Allies' first major victory against the Japanese, it did not radically change the course of the war; instead it was the cumulative effects of the battles of the Coral Sea and Midway that reduced Japan's ability to undertake major offensives.
In addition Midway paved the way for the landings on Guadalcanal, and the prolonged attrition of the Solomon Islands campaign, both of which finally allowed the Allies to take the strategic initiative and swing onto the offensive for the rest of the Pacific War.
Finally, Midway bought the United States valuable time until the first of the new "Essex"-class fleet carriers became available at the end of 1942.
The battle also showed the worth of pre-war naval cryptanalysis and intelligence-gathering.
These efforts continued and were expanded throughout the war in both the Pacific and Atlantic theaters.
Successes were numerous and significant.
For instance, Navy cryptanalysis made possible the shooting down of Admiral Yamamoto's airplane.
Some authors have stated heavy losses in carriers and veteran aircrews at Midway permanently weakened the Imperial Japanese Navy.
Parshall and Tully, however, have stated that the losses in veteran aircrew, while heavy (110, just under 25% of the aircrew embarked on the four carriers), were not crippling to the Japanese naval air-corps as a whole: the Japanese navy had some 2,000 carrier-qualified aircrew at the start of the Pacific war.
A few months after Midway, the JNAF sustained similar casualty rates at both the Battle of the Eastern Solomons and Battle of Santa Cruz, and it was these battles, combined with the constant attrition of veterans during the Solomons campaign, which were the catalyst for the sharp downward spiral in operational capability.
However, the loss of four large fleet carriers, and over 40% of the carriers' highly trained aircraft mechanics and technicians, plus the essential flight-deck crews and armorers, and the loss of organizational knowledge embodied by such highly trained crew, were heavy blows to the Japanese carrier fleet.
After the battle and were the only large carriers of the original Pearl Harbor strike-force left for offensive actions.
Of Japan's other carriers, , which was not commissioned until early 1944, would be the only fleet carrier worth teaming with "Shōkaku" and "Zuikaku"; and "Zuihō" were light carriers, while and , although technically classified as fleet carriers, were second-rate ships of comparatively limited effectiveness.
By the time of the Battle of the Philippine Sea, the Japanese had somewhat rebuilt their carrier forces in terms of numbers, but their planes, which by then had become obsolescent, were largely flown by inexperienced and poorly trained pilots.
In the time it took Japan to build three carriers, the U.S. Navy commissioned more than two dozen fleet and light fleet carriers, and numerous escort carriers.
By 1942 the United States was already three years into a shipbuilding program mandated by the Second Vinson Act, intended to make the navy larger than Japan's.
The greater number of USN aviators survived the Battle of Midway and subsequent battles of 1942, and combined with growing pilot training programs, the US accumulated a large number of skilled pilots to complement its material advantages in ships and planes.
An armoured flight deck is an aircraft carrier flight deck that incorporates substantial armour in its design.
Comparison is often made between some of designs of the Royal Navy (RN) and the United States Navy (USN).
The two navies followed differing philosophies in the use of armour on carrier flight decks starting with the design of the RN's "Illustrious" class and ending with the design of the "Midway" class, when the USN also adopted armoured flight decks.
The two classes most easily compared are the RN's "Illustrious" class and "Implacable" class and their nearest USN contemporaries, the "Yorktown" and "Essex" classes.
The "Illustrious" class followed the "Yorktown" but preceded the "Essex", while the "Implacable" class design predated the "Essex" but these ships were completed after the lead ships of the "Essex" class.
The development of armoured flight deck carriers proceeded during World War Two and before the end of World War Two, both the USN, with USS "Midway", and the Imperial Japanese Navy (IJN), with "Taiho" and "Shinano", would also commission armoured flight deck carriers, while all USN fleet aircraft carriers built since 1945 feature armoured flight decks.
The remainder of the IJN carrier force during World War II had unarmoured flight decks just like the Yorktown and Essex classes of the USN. 
The "Independence"-class aircraft carriers were a class of light carriers built for the United States Navy that served during World War II.
This class of ship were constructed as a result of the interest of President Franklin D. Roosevelt.
With war looming, the former Secretary of the Navy noted no new fleet aircraft carriers were expected to be completed before 1944.
He proposed to convert some of the many cruisers then under construction to carriers.
Studies of cruiser-size aircraft carriers had shown the type had serious limitations, and on October 13, 1941, the General Board of the United States Navy replied that such a conversion showed too many compromises to be effective.
Undeterred, President Roosevelt ordered another study.
On Oct 25th a second study from BuShips reported that the cruiser conversion aircraft carrier would result in a vessel of lesser capability, but they would be available much sooner.
Following the December 1941 Pearl Harbor attack the need for more carriers became urgent.
In January 1942 The Navy responded by greatly accelerating construction of the 34,000 ton Essex class aircraft carriers, and ordering the conversion of a Cleveland class light cruiser then under construction to completion as a light aircraft carrier, becoming the USS "Belleau Wood" (CVL-24). 
In September 1945, "Bunker Hill" reported for duty with the Operation Magic Carpet fleet, returning veterans from the Pacific.
She remained on this duty as a unit of TG 16.12 until January 1946, when she was ordered to Bremerton for deactivation.
She was decommissioned into reserve on 9 January 1947.
While she was laid up in mothballs, she was reclassified three times, becoming CVA – 17 in October 1951, CVS – 17 in August 1953, and AVT – 9 in May 1959, with the latter designation indicating that any future commissioned operations would be as an "Auxiliary Aircraft Landing Training Ship".
As all "Essex"-class carriers survived the war, "Bunker Hill" was surplus to the needs of the navy.
She and , which also had sustained severe damage from an aerial attack, were the only aircraft carriers in the "Essex"-class that did not experience any active duty after the end of World War II, despite their being repaired.
Stricken from the Naval Vessel Register in November 1966, "Bunker Hill" was used as a stationary electronics test platform at the Naval Air Station North Island, San Diego, during the 1960s and early 1970s.
"Bunker Hill" was sold for scrapping in May 1973.