Question: When did the Mk IIa series 1 Hurricanes first enter service?
About the article "Battle of Britain", section "Fighters".

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 Hurricane Mk I First production version, with fabric-covered wings, a wooden two-bladed, fixed-pitch propeller, powered by the 1,030 hp (768 kW) Rolls-Royce Merlin Mk II or III engines and armed with eight .303 in (7.7 mm) Browning machine guns.
Produced between 1937 and 1939.
Hurricane Mk I (revised) A revised Hurricane Mk I series built with a de Havilland or Rotol constant speed metal propeller, metal-covered wings, armour and other improvements.
In 1939, the RAF had taken on about 500 of this later design to form the backbone of the fighter squadrons.
Hurricane Mk IIA Series 1 Hurricane Mk I powered by the improved Merlin XX engine.
This new engine used a mix of 30 per cent glycol and 70 per cent water.
Pure glycol is flammable, so not only was the new mix safer, but the engine also ran approximately 70 °C cooler, which gave longer engine life and greater reliability.
The new engine was longer than the earlier Merlin and so the Hurricane gained a 4.5 in "plug" in front of the cockpit, which made the aircraft slightly more stable due to the slight forward shift in centre of gravity.
First flew on 11 June 1940 and went into squadron service in September 1940.
Hurricane Mk IIB (Hurricane IIA Series 2) The Hurricane II B were fitted with racks allowing them to carry two 250 lb or two 500 lb bombs.
This lowered the top speed of the Hurricane to 301 mph (484 km/h), but by this point mixed sweeps of Hurricanes protected by a fighter screen of Hurricanes were not uncommon.
The same racks would allow the Hurricane to carry two 45-gallon (205 l) drop tanks instead of the bombs, more than doubling the Hurricane's fuel load.
Hurricane Mk IIA Series 2 was equipped with new and slightly longer propeller spinner and new wing mounting 12 x .303 in (7.7 mm) Browning machine guns.
The first aircraft were built in October 1940 and were renamed Mark IIB in April 1941.
Hurricane Mk IIB Trop.
For use in North Africa the Hawker Hurricane Mk IIB (and other variants) were tropicalised.
They were fitted with Vokes and Rolls Royce engine dust filters and the pilots were issued with a desert survival kit, including a bottle of water behind the cockpit.
Hurricane Mk IIC (Hurricane Mk IIA Series 2) Hurricane Mk IIA Series 1 equipped with new and slightly longer propeller spinner and fully replaced the machine-gun armament with four 20 mm (.79 in) Hispano Mk II cannons, two per side.
Hurricane IIA Series 2 became the Mk IIC in June 1941, using a slightly modified wing.
The new wings also included a hardpoint for a or bomb, and later in 1941, fuel tanks.
By then performance was inferior to the latest German fighters, and the Hurricane changed to the ground-attack role, sometimes referred to as the Hurribomber.
The mark also served as a night fighter and intruder.
Hurricane Mk IID Hurricane Mk IIB conversion armed with two 40 mm (1.57 in) anti-tank autocannons in a gondola-style pod, one under each wing and a single Browning machine gun in each wing loaded with tracers for aiming purposes.
The first aircraft flew on 18 September 1941 and deliveries started in 1942.
Serial built aircraft had additional armour for the pilot, radiator and engine, and were armed with a Rolls-Royce gun with 12 rounds, later changed to the 40 mm (1.57 in) Vickers S gun with 15 rounds.
The outer wing attachments were strengthened so that 4G could be pulled at a weight of 8,540 lb (3,874 kg).
The weight of guns and armour protection marginally impacted the aircraft's performance.
These Hurricanes were nicknamed "Flying Can Openers", perhaps a play on the No.
6 Squadron's logo which flew the Hurricane starting in 1941.
Hurricane Mk IIE Another wing modification was introduced in the Mk IIE, but the changes became extensive enough that it was renamed the Mk IV after the first 250 had been delivered.
Hurricane Mk T.IIC Two-seat training version of the Mk.
IIC.
Only two aircraft were built for the Persian Air Force.
Hurricane Mk III Version of the Hurricane Mk II powered by a Packard-built Merlin engine, intending to provide supplies of the British-built engines for other designs.
By the time production was to have started, Merlin production had increased to the point where the idea was abandoned.
Hurricane Mk IV The last major change to the Hurricane was the introduction of the "universal Wing", a single design able to mount two 250 lb or 500 lb (110 or 230 kg) bombs, two 40 mm (1.57 in) Vickers S guns, drop tanks or eight "60 pounder" RP-3 rockets.
Two .303 in Brownings were fitted to aid aiming of the heavier armament.
The new design also incorporated the improved Merlin 24 or 27 engines of 1,620 hp (1,208 kW), equipped with dust filters for desert operations.
The Merlin 27 had a redesigned oil system that was better suited to operations in the tropics, and which was rated at a slightly lower altitude in keeping with the Hurricane's new role as a close-support fighter.
The radiator was deeper and armoured.
Additional armour was also fitted around the engine.
Hurricane Mk V The final variant to be produced.
Only three were built and it never reached production.
This was powered by a Merlin 32 boosted engine to give 1,700 hp at low level and was intended as a dedicated ground-attack aircraft to use in Burma.
All three prototypes had four-bladed propellers.
Speed was 326 mph (525 km/h) at 500 ft, which is comparable with the Hurricane I despite being one and a half times as heavy.
Hurricane Mk X Canadian-built variant.
Single-seat fighter and fighter-bomber.
Powered by a 1,300 hp (969 kW) Packard Merlin 28.
Eight 0.303 in (7.7 mm) machine guns mounted in the wings.
In total, 490 were built.
Hurricane Mk XI Canadian-built variant.
150 were built.
Hurricane Mk XII Canadian-built variant.
Single-seat fighter and fighter-bomber.
Powered by a 1,300 hp (969 kW) Packard Merlin 29.
Initially armed with 12 0.303 in (7.7 mm) machine guns, but this was later changed to four 20 mm (.79 in) cannon.
Hurricane Mk XIIA Canadian-built variant.
Single-seat fighter and fighter-bomber.
Powered by a 1,300 hp (969 kW) Packard Merlin 29, armed with eight 0.303 in (7.7 mm) machine guns.
Sea Hurricane Mk IA The Sea Hurricane Mk IA was a Hurricane Mk I modified by General Aircraft Limited.
These conversions numbered approximately 250 aircraft.
They were modified to be carried by CAM ships (catapult armed merchantman), whose ships' crews were Merchant Marine and whose Hurricanes were crewed and serviced by RAF personnel, or Fighter Catapult Ships, which were Naval Auxiliary Vessels crewed by naval personnel and aircraft operated by the Fleet Air Arm.
These ships were equipped with a catapult for launching an aircraft, but without facilities to recover them.
Consequently, if the aircraft were not in range of a land base, pilots were forced to bail out or to ditch.
Both of these options had their problems — there was always a chance of striking part of the fuselage when bailing out and a number of pilots had been killed in this way.
Ditching the Hurricane in the sea called for skill as the radiator housing acted as a water brake, pitching the nose of the fighter downwards when it hit the water, while also acting as very efficient scoop, helping to flood the Hurricane so that a quick exit was advisable before the aircraft sank.
Then the pilot had to be picked up by the ship.
More than 80 modifications were needed to convert a Hurricane into a Sea Hurricane, including new radios to conform with those used by the Fleet Air Arm and new instrumentation to read in knots rather than miles per hour.
They were informally known as "Hurricats".
The majority of the aircraft modified had suffered wear-and-tear serving with front line squadrons, so much so that at least one example used during trials broke up under the stress of a catapult launching.
CAM Sea Hurricanes were launched operationally on eight occasions and the Hurricanes shot down six enemy aircraft for the loss of one Hurricane pilot killed.
The first Sea Hurricane IA kill was an FW 200C "Condor", shot down on 2 August 1941.
Sea Hurricane Mk IB Hurricane Mk I version equipped with catapult spools plus an arrester hook.
From July 1941 they operated from and from October 1941, they were used on Merchant aircraft carrier (MAC ships), which were large cargo vessels with a flight deck fitted, enabling aircraft to be launched and recovered.
A total of 340 aircraft were converted.
The first Sea Hurricane IB kill occurred on 31 July 1941 when Sea Hurricanes of 880 squadron FAA operating from HMS "Furious" shot down a Do 18 flying-boat.
Sea Hurricane Mk IC Hurricane Mk I version equipped with catapult spools, an arrester hook and the four-cannon wing.
From February 1942, 400 aircraft were converted.
The Sea Hurricane IC used during "Operation Pedestal" had their Merlin III engines modified to accept 16 lb boost, and could generate more than 1400 hp at low altitude.
Lt. R. J. Cork was credited with five kills while flying a Sea Hurricane IC during "Operation Pedestal".
Sea Hurricane Mk IIC Hurricane Mk IIC version equipped with naval radio gear; 400 aircraft were converted and used on fleet carriers.
The Merlin XX engine on the Sea Hurricane generated 1460 hp at 6,250 ft and 1435 hp at 11,000 ft. Top speed was 322 mph at 13,500ft and 342 mph at 22,000 ft. Sea Hurricane Mk XIIA Canadian-built Hurricane Mk XIIA converted into Sea Hurricanes.
Hillson F.40 (a.k.a.
F.H.40) A full-scale version of the Hills & Son Bi-mono slip-wing Biplane/monoplane, using a Hawker Hurricane Mk I returned from Canada as RCAF ser no 321 (RAF serial L1884).
Taxi and flight trials carried out at RAF Sealand during May 1943, and at the Aeroplane and Armament Experimental Establishment, Boscombe Down from September 1943.
The upper wing was not released in flight before the programme was terminated due to poor performance.
Hurricane Photo Reconnaissance In Egypt, the Service Depot at Heliopolis converted several Hurricanes Is for the role.
The first three were converted in January 1941.
Two carried a pair of F24 cameras with 8-inch focal length lenses.
The third carried one vertical and two oblique F24s with 14-inch focal length lenses mounted in the rear fuselage, close to the trailing edge of the wing, and a fairing was built up over the lenses aft of the radiator housing.
A further five Hurricanes were modified in March 1941 while two were converted in a similar manner in Malta during April 1941.
During October 1941 a batch of six Hurricane IIs was converted to PR Mark II status and a final batch, thought to be of 12 aircraft, was converted in late 1941.
The PR Mark II was said to be capable of slightly over 350 mph (563 km/h) and was able to reach 38,000 ft (11,600 m).
Hurricane Tac R For duties closer to the front lines some Hurricanes were converted to Tactical Reconnaissance (Tac R) aircraft.
An additional radio was fitted for liaison with ground forces who were better placed to direct the Hurricane.
Some Hurricane Tac R aircraft also had a vertical camera fitted in the rear fuselage, so to compensate for the extra weight either one or two Brownings or two cannons would be omitted.
Externally these aircraft were only distinguishable by the missing armament.  
 Royal Australian Air Force * No.
1 Squadron RAAF - Bulldog Mk.
IIA * No.
2 Squadron RAAF - Bulldog Mk.
IIA Royal Danish Air Force Estonian Air Force Finnish Air Force - Bulldog Mk.
IVA  Imperial Japanese Navy Air Service Latvian Air Force  (now Thailand) Royal Siamese Air Force Two bought for comparison purposes Spanish Republican Air Force Royal Swedish Air Force Two remaining Bulldog Mk.
IIA donated to Finland 1939 Royal Air Force * No.
3 Squadron RAF - Bulldog Mk.
II and Bulldog Mk.
IIA * No.
17 Squadron RAF - Bulldog Mk.
II and Bulldog Mk.
IIA * No.
19 Squadron RAF - Bulldog Mk.
IIA * No.
23 Squadron RAF - Bulldog Mk.
IIA * No.
24 Squadron RAF - One two-seat Bulldog was used by No.
24 (Communications) Squadron.
* No.
29 Squadron RAF - Bulldog Mk.
IIA * No.
32 Squadron RAF - Bulldog Mk.
IIA * No.
41 Squadron RAF - Bulldog Mk.
IIA * No.
54 Squadron RAF - Bulldog Mk.
IIA * No.
56 Squadron RAF - Bulldog Mk.
IIA * No.
111 Squadron RAF - Bulldog Mk.
IIA * No.
3 Flying Training School * No.
5 Flying Training School * Central Flying School RAF * RAF College, Cranwell United States Navy - The U.S. Navy purchased two aircraft between 1929–1930.
The aircraft were used for test and trials.  
 Airworthy  Hurricane Mk IIa "DR393" (registered ZK-TPK) was originally a Mk I "P3351".
It was delivered to the RAF, and crashed near Prestwick on 21 July 1940.
Rebuilt as Mk IIa "DR393" the aircraft was delivered to the Soviet Air Force in May 1941, serving for about two years before again crashing near Murmansk, Russia in 1943.
The hulk was eventually restored as a Mk IIc in the UK from 1992 to 1995.
It was transferred to New Zealand in 1995, and acquired by the New Zealand Fighter Pilots Museum in Wanaka, New Zealand.
Its first flight after restoration occurred in Christchurch on 12 January 2000.
Bought by Jan Roozan at auction and shipped to France on February 10, 2013.
Arrived at Le Havre on March 30 and taken to Aero Restauration Service at Dijon for re-assembly.
Registered as F-AZXR on May 14, 2013. 
 Mk I = introduced in 1918 and declared obsolete in 1919 Mk II = the first model introduced in 1938 with a large baseplate Mk II* = the 1938 version intended for use with the Universal (Bren Gun) Carrier Mk II** = a second version for use with the Universal Carrier Mk II*** = version for use by infantry at platoon level and fitted with a large baseplate Mk III = version used as a smoke launcher for tanks Mk IV = limited production run and did not enter service Mk V = not manufactured Mk VI = not manufactured Mk VII = for use on Universal Carriers Mk VII* = for use by airborne forces, having a shorter barrel ( = 36 cm) and a baseplate replaced with a spade-like plate Mk VII** = infantry use with long barrel and spade-like baseplate Mk VIIA = Indian Army model Mk VIII = another short-barrelled version for the airborne forces  
  Firefly I / FR.I  Two variants of the Mk I Firefly were built; 429 "fighter" "Firefly F Mk I"s, built by Fairey and General Aircraft Ltd, and 376 "fighter/reconnaissance" Firefly "FR Mk I"s (which were fitted with the ASH detection radar).
The last 334 Mk Is built were upgraded with the 1,765 hp (1,316 kW) Griffon XII engine.
Firefly NF.
Mk II  Only 37 Mk II Fireflies were built, all of which were night fighter Firefly NF Mk IIs.
They had a slightly longer fuselage than the Mk I and had modifications to house their airborne interception (AI) radar.
Firefly NF.
Mk I  The NF.II was superseded by the Firefly NF Mk I "night fighter" variant.
Firefly T. Mk 1  Twin-cockpit pilot training aircraft.
Post-war conversion of the Firefly Mk I.  Firefly T. Mk 2  Twin-cockpit armed operational training aircraft.
Post-war conversion of the Firefly Mk I.  Firefly T. Mk 3  Used for Anti-submarine warfare training of observers.
Postwar conversion of the Firefly Mk I.  Firefly TT.
Mk I  Postwar, a small number of Firefly Mk Is were converted into target tug aircraft.
Firefly Mk III  A Firefly Mk III was proposed, based on the Griffon 61 engine, but never entered production.
Firefly Mk IV  The Firefly Mk IV was equipped with the 2,330 hp (1,740 kW) Griffon 72 engine and first flew in 1944, but did not enter service until after the end of the war.
Firefly FR.
Mk 4  Fighter-reconnaissance version based on the Firefly Mk IV.
Firefly Mk 5  Firefly NF.
Mk 5  Night fighter version based on the Firefly Mk 5.
Firefly RF.
Mk 5  Reconnaissance fighter version based on the Firefly Mk 5.
Firefly AS.
Mk 5  The later Firefly AS.
Mk 5 was an anti-submarine aircraft, which carried American sonobuoys and equipment.
Firefly Mk 6  Firefly AS.
Mk 6  The Fairefly AS.
Mk 6 was an anti-submarine aircraft, which carried British equipment.
Firefly TT.
Mk 4/5/6  Small numbers of AS.4/5/6s were converted into target tug aircraft.
Firefly AS.
Mk 7  The Firefly AS.
Mk 7 was an anti-submarine aircraft, powered by a Rolls-Royce Griffon 59 piston engine.
Firefly T. Mk 7  The Firefly T. Mk 7 was an interim ASW training aircraft.
Firefly U. Mk 8  The Firefly U. Mk 8 was a target drone aircraft; 34 Firefly T.7s were diverted on the production line for completion as target drones.
Firefly U. Mk 9  The Firefly U. Mk 9 was a target drone aircraft; 40 existing Firefly Mk AS.4 and AS.5 aircraft were converted to this role.