I will keep this thread updated with the latest speed class definitions.

Class 1: Below 250 KTAS

Avro Anson
Beechcraft D-18/C-45
Bristol Blenheim
Consolidated PB4Y
Curtiss C-46
Douglas DC-3/C-47
Douglas SBD
Gloster Gladiator
Grumman F3F
Grumman TBF/TBM
Lockheed Hudson
Mitsubishi A5M
Mitsubishi G4M1

Class 2: 250 - 300 KTAS

CAC Boomerang
Boeing B-17
Brewster F2A
Bristol Beaufighter
Consolidated B-24
Curtiss P-36
Grumman F4F/FM-2
Hawker Hurricane (Mk I)
Lockheed PV-2
Nakajima B5N
Martin B-26
Mitsubishi A6M
North American B-25

Class 3: 300 - 350 KTAS

Bell P-39/400
Boeing B-29A
Curtiss P-40 (Not Q)
DeHavilland DH.98 Mosquito
Douglas A-26
Hawker Hurricane (Mk IV)
Grumman F6F
Nakajima B6N
Nakajima Ki-43
Supermarine Spitfire (Mk IA)
Supermarine Seafire (IIC, LIII)

Class 4: Above 350 KTAS

Chance-Vought F4U
Grumman F7F
Lockheed P-38 (Not K)
Mitsubishi J2M
North American P-51 (Not H)
Republic P-47 (Not M or N)