The DC-9 was the start of a long series of aircraft, strongly varying in length and also engine types, ultimately ending with the Boeing 717. It was the first twin jet airliner in the west with a T-tail and engines attached to the rear fuselage. Its nose is based on the DC‑8, without the air intakes at the sides of the nose, but still with the single eyebrow window on each side. Also, the nose gear is more to the front. Other features are small, slender wings, a double bubble fuselage cross section and an air intake at the base of the vertical stabiliser.

The different versions of the DC-9, MD-80, MD-90 and Boeing 717 can be recognised by looking at

• the length of the fuselage
• the number of overwing emergency exits
• the shape of the engine nacelles
• the shape of the tail cone
• the shape of the vertical stabiliser
• the presence of a strake at the side of the nose
• the presence of a large cargo door in the left forward fuselage
• the lack of cabin windows
• the wing span
• the presence of a small wing fence in the leading edge

It all started with the DC-9 Series 10, although an even shorter Series 5 was initially planned. This short-body version has only one overwing emergency exit, although some aircraft seem to have received two. Typical is the small wing fence in the leading edge of the wings, beside a longer one underneath the wings. The wings have no leading edge slats. For the rest it has the DC-9 characteristics like the pointed tail cone, no bullet fairing on top of the vertical fin and slim engine nacelles without strakes.

The DC-9-11, -12, -14 and 15 only differ in exact engine type, maximum takeoff weight and performance.

Douglas immediately made cargo versions of the short DC-9. They are actually con­vertible versions with cabin windows, but also a large cargo door in the left forward fuselage. There are two versions, the DC-9-15MC (multiple change) and DC-9-15RC (rapid change), that are externally the same.

Scandinavian Airlines System (SAS) wished a DC-9 Series 10 that could operate from shorter runways. The Series 20 combine the fuselage of the Series 10 with the wings of the Series 30. These wings have leading edge slats, and a slightly larger span (which is barely notice­able). Due to the slats, the small wing fence was removed.

Soon, the Series 10 were considered too small, especially when Boeing introduced the 737. Hence, the DC-9 Series 30 is 4.5 metres longer than the Series 10 and 20. It has two overwing emergency exits. The wings have a slightly wider span, but this is not easy to see. Better visible are the leading edge slats, which caused the small leading edge fence to disappear.

The three subversion basically only differ in engine version, weights and performance, although the DC-9-34 has a 1.25 degrees larger wing incidence than other version. However, this is in practice invisible.

Like for the shortest DC-9, the Series 30 also has mixed passenger/cargo versions, which have many cabin windows and a large cargo door in the left front fuselage. They have suffixes CF, for convertible freighter, and RC, for rapic change. The DC-9-33RC and DC-9-34CF have the same larger wing incidence as the DC-9-34.

The USAF had its own version of the DC-9-32CF, which was designated C-9A Nightingale. A similar version for the US Navy and US Marines was called C-9B Skytrain II. The USAF also used the VC-9C, a VIP version of the DC-9-32.

There were also dedicated freighter versions of the DC-9 Series 30, namely DC‑9‑32AF and DC-9-33AF, of all freight. They were built from the start with holes for cabin windows, which were then filled with metal plugs. Mutually, they are different in engines and wing incidence, but nothing external.

Another version developed specifically for SAS was the DC-9 Series 40. This DC-9-41 is just two metres longer than the Series 30 and also has two emergency exits, so is not easy to distinguish from it. But side-by-side it is obvious that the Series 40 is longer.

The last of the original DC-9 family was a further stretch, 2.5 m compared to the Series 40 and 4.5 to the Series 30. It still has two overwing emergency exits. Apart from the length you can also recognise the Series 50 by the strake at the side of the nose.

The fuselage of the DC-9 could even be further stretched, 4.5 metres compared to the Series 50. These DC-9 Super 80 versions have slightly wider diameter engines, but in similar shaped nacelles. These nacelles have a strake along the lower side though. Also the strake at the side of the nose remains. The vertical stabiliser's leading edge has a slightly bump at the top. Like most DC-9s the long MD-80s have two overwing emergency exits, but also a large one on the left side, in front of the engine. At first the Super 80s had the same pointed tail cone at the DC-9, but later they were built with the flat tail cone of the MD-87.

The subvariants DC-9-81, DC-9-82 and DC-9-83 differ in exact engine version and perfor­mance only.

After the merger with McDonnell, McDonnell-Douglas continued production and devel­opment, but under the marketing name MD-81, MD-82 and MD-83. The MD-88 was a version with a digital cockpit, but is externally the same as the other three subtypes.

In the 1980s several engine manufacturers developed Ultra High By-pass (UHB) engines with open fan rotors. These were tested on multiple aircraft, including the MD-80. These testbeds had the new engine in place of the left JT8D. Both General Electric and Allison made use of this converted MD-81.

Ultra High By-pass engines, or propfans, were extensively tested in the 1980s, but did not become widely adopted. This MD-80 was one of the demonstrators. (photo: Andrew Thomas/WikiMedia)

Next to making only longer and longer versions of the original DC-9, McDonnell-Douglas made a shorter version of the MD-80, called MD-87. Compared to the longer MD-80s, the MD-87 has an extended top of the vertical stabiliser and a longer strake on the nacelle, but no large emergency exit in front of the left engine. Also, this version was fitted with a flat tail cone from the start.

While not as popular as the Boeing 737 Classic and Next Generation, there are a few MD-80 freighter conversions. This is done by AEI, after which they are designa­ted MD-81SF, MD-82SF, MD-83SF or MD‑88SF (special freighter). They have of course a large cargo door, in the left front fuselage, and no cabin windows anymore.

The lack of cabin windows is a good clue that this is an MD-80SF, even though you cannot see the cargo door. (photo: Jeroen Stroes Aviation Photography/WikiMedia)

In a last attempt to keep up with Boeing and Airbus, McDonnell-Douglas developed the MD‑90, basically a slightly (1.5 m) stretched MD-88 with V2500 engines and an MD-87 tail. This version was called MD-90-30, as more subvariants were to come. The V2500 engines require wider diameter nacelles than the original JT8Ds. They still have a strake along the lower side of them. Also they have no thrust reverser fairings: the whole rear of the nacelle slides rearward to open the reverser.

When McDonnell-Douglas was taken over by Boeing it was working on a successor of the MD-87 as a true replacement of the DC-9-30. This aircraft, designated MD-95, was to be lighter and thus cheaper to operate than the MD-87 which is simply a shrunken MD-80. Instead of cancelling the project Boeing continued the development as the Boeing 717.

The aircraft is best recognised by the shape of the engine nacelles, because it has larger diameter BR700 series engines instead of one of the JT8D family. These nacelles are longer than that of the MD-90, but have no thrust reverser fairings either. It also has the flat tail cone of the MD-80 series and an MD-87 like tail.