In order to make its piston twin airplanes easier to fly after an engine failure, Cessna devel­oped one with both engines on the fuselage centreline, the model 336 Skymaster. The engine in the nose has a tractor (or pull) propeller, like on most prop airplanes. The other is in the rear fuselage, getting its air from an intake on top of the rear fuselage, and has a pusher propeller. Hence, the plane is often referred to as the Cessna 'push-pull'. To allow such a configuration, the Skymaster has two tail booms extending from the trailing edges of the straight, high-mounted, strut-braced wings. At both ends are swept vertical fins up and down. The top fin is larger than the bottom fin. In between the tail booms is a low single horizontal stabiliser. Later it was developed into the Cessna 337 Super Skymaster, that also had a military variant, the O-2.

The different versions of the Cessna 336/337 family can be distinguished by:

• whether the gear is retractable or not
• the size of the cowl flaps of the rear engine
• the number and size of cabin windows
• the presence of pylons under the wings
• the number and shape of the windows in the cabin door
• the shape of the wing tips
• the presence of a blade and loop antenna on top of the fuselage
• the presence of prop spinners
• the presence of wing fences
• the location of the landing lights

Cessna thought there was a market for a four seat version of the model 337. Hence it developed model 327. Apart from being smaller, the Cessna 327 has cantilever, non-braced wings as main external difference. Unfortunately for Cessna, there was no commercial interest, to just one was built.

The fixed, all single wheel landing gear is the main recognition point of the Cessna 336 Skymaster. Still, it may not always be directly obvious if the gear is fixed or just extended. Then wheel pants are of course a good help, and otherwise look for the small door be­hind the nose gear that remains out when the gear is down. Also, sometimes there are holes in the main gear housing visible. The 336 has vertical fins down that are about half the size of the fins up. The cooling intake for the rear engine is slightly curved. Common with the first model 337s are the three large side win­dows, reduring in size further aft.

The wheel pants help a lot identifying this as a Cessna 336 Skymaster, and not a 337 Super Skymaster. Otherwise you could have noticed there is no visible nose gear door, or longer bottom tail fins. (photo: Spartan7W/WikiMedia, modified)

Starting with the 1965 model, Cessna significantly improved the Skymaster to the model 337 Super Skymaster. It is the first to have a retractable gear. The nose gear goes forward and the main gear backward, under the rear engine. When the gear is extended, a small door is still visible behind the nose gear. Other changes include a narrow rectangular, non-curved rear cooling intake on top of the rear fuselage, shorter vertical fins down (about a third of the height of the stabilisers up), slightly different engines cowlings, and many hardly noticeable smaller differences.

The 337A, 337B, 337C & 337D mostly differ internally for the 337 and each other. The 337A is supposed to have a rotary-type door latch, but that can just as well be placed on the 337. The 337D has a smaller chord elevator, but it is doubtful whether that is visible. All variants have in common straight, non-cambered taper on the wing tips and landings lights in the leading edge of the wings.

The 1970 model year brought the Cessna 337E, which has cambered wing tips as main external difference. This means that the top and bottom of the wings both curve down a bit at the tip. Beware however that the Robertson STOL modification also involve a wing tip camber, but slightly more. Additionally, the modificition bring wings fences and a recontoured wing leading edge.

Initially, the Cessna 337F was externally not different from the E version, but that changed with the 1972 model, when the landing lights moved from the wing leading edge to the nose, under the propeller spinner.

The Cessna 337E is the first Super Skymaster version that was built under license by Reims Aviation in France. Its designation is Reims F337E. The Reims F337F also exists.

With the Cessna 337G introduced in 1973 the fuselage looks a bit different. First of all, the cabin windows were reduced in size, in particular the two rear ones. Secondly, the vertically hinged cabin door on the right is now split horizontally, opening up and down. The lights in the wing tips are larger than previously, and flush with the tip contour. Other differences are much smaller and difficult to see.

The Cessna 337H is externally not different from the G model.

From the 1967 B model on, Cessna offered a normal aspirated piston version, and a turbo-charged one. These have a T prefix. They can best be distinguished from the non-turbo versions by the size of the cowl flap/door or the rear engine. The turbo-charged one is larger (square) than the non-turbo one (rectangular).

The Cessna 337E and 337F were also offered with a turbocharged engine, so they have the large rear engine cowl flap. For the rest they are the same as the normally aspirated variants.

The turbocharged version of the G and H models additionally have a pressurised cabin. Hence, the window configuration is different: the cockpit has a small triangular side window, and the cabin has four nearly square windows on each side. On the right side, there are two cabin windows in the upper part of the door.

Cessna uses model numbers P337G and P337H, although T337G is also sometimes used. They are extermally the same. The same applies to the Reims built examples.

Unlike for the G model Cessna made a dedicated turbocharged variant of the H model, called Cessna T337H. Like the other versions with a turbocharged engine it has large rear engine cowl flaps. It also has the other elements of the standard 337H, such as the two small rear cabin windows, and the split cabin door.

The Cessna T337H-SP only has a slightly different engine model.

One large rectangular side window, two square ones and a large, square open cowl flap, so this must be a Cessna T337H! (photo: Bob Adams/WikiMedia)

The new observation and light attach airplane for Viernam is a special vesion of the Cessna 337B. It received dedicated model number 337M. On the outside the most obvious differences are the window in the bottom of the cabin door and below the front window on the right side, a large blade antenna on top of the fuselage, a half loop antenna above the front windows and two pylons under each wing. Additionally, in most cases the O-2A has no propeller spinners.

Some dozen civil Cessna 337s were conver­ted to O-2B, with loudspeakers under the right aft cabin window, a leaflet dispenser, a large blade antenna above the cockpit and a half-loop antenna above the front windows. In many cases they lost the propeller spinners. Unlike the O-2A, the O-2B has no underwing pylons and no windows in the bottom of the right cabin door.

The US military had plans for an improved version of the O-2, with turboprop engines and a tandem cockpit. One O-2A was converted to the O-2T concept aircraft, receiving Cessna model number 348. This has the turboprop engines, but not the redesign fuselage. The front cowling with exhausts in the left and right bottom corners is the main external difference compared to the regular O-2.

Similar to the Voltaero Cassio S, the Ampaire Electric EEL is a test airplane for electrical propulsion systems, and a conversion of a Cessna 337. It retains two engines and props though, with only the front propeller being driven by an electric motor. The power comes from batteries in a belly pod. Other modifications include large triangular dorsal fins and large winglets.

Reims Aviation of France built many Cessnas under license, including the 337, but there is only one variant exclusively built by the French, the FTB337G. It is a military version of the F337G, with four pylons under the wings like on the O-2A. Additionally, this version has two small rectangular windows in the lower part of the cabin door. Finally, it has the full Robertson STOL kit, including different wing tips, wing fences, re-contoured wing leading edges and short, curved dorsal fins.

From this angle you can well see the wing modifications that are part of the Robertson STOL kit, in particular the wing fences. Also note the short curved dorsal fins. The wing pylons are not visible, but their presence is given away by the rocket launcher attached to one. (photo: Alan Wilson/WikiMedia) 

One typical feature of the Reims FTB337G are the small rectangular windows in the lower part of the cabin door on the right side. (photo: Pedro Aragão/WikiMedia)

This conversion turns a standard civil Cessna 337 more or less into an O-2A. Like the O-2A the Summit Sentry O2-337 has four pylons under the wings, a blade antenna and half loop antenna on top of the fuselage and observations windows on the right side. You can only distinguish it from the real O-2A if the rear window arrangement of the original 337 is retained.

Thanks to its window configuration and curved dorsal fins, this Cessna T337H-SP reveals itself as A summit Sentry conversion and not an O‑2A. (photo: Gerry Stegmeier/WikiMedia)