The Learjet is close to the synonym for a (small) private jet. That is not only because William Lear was one of the first to develop such an aircraft, but also due to its outstanding performance: even though the first Learjet models had nearly straight wings, they had a fighter-like performance. At first the Learjets were marketed and built by Lear Jet Corp. After a brief period being named Lear Jet Industries Inc. the company merged with Gates Aviation Corp. to create Gates Learjet Corp. Later in the 1980s it became Learjet Corp. again. This stayed so until Bombardier acquired the Learjet programme in 1990.

The Learjets are first characterised by their cockpit windows. Until the Learjet 55 all had two-piece curved cockpit windows. While the windows have changed a bit on later models, they are still very recognisable. Another feature of especially the early Learjets is the relatively small tail. The appearance is thus like of the Lockheed Starfighter.

To differentiate between the different subtypes there is a lot to look at

• the length of the fuselage
• the shape of the cabin windows
• the number of the cabin windows
• the shape of the cockpit windows and window frames
  
• the shape of the engine nacelles
• the number of wheels on the main landing gear
• the shape of the wings
• the presence of tip tanks
• the presence of winglets
• the presence and shape of ventral fins
• the wing-fuselage attachment

The first Learjets, including model 23, have in common the large oval cabin windows (one on the left, two on the right) and a so-called bullet fairing in front of the horizontal tail plane. Specific for the Learjet 23 are the  cockpit window frames which are flat, including the one between the two windows. The window defrosters are slightly separated from the central frame. Also, the emergency exit in the second large window on the right has no handle on the outside like on the 24.

We have put these model Learjets under one heading because there are no known external differences. They still have the large oval cabin windows (one on the left, two on the right) and a so-called bullet fairing in front of the horizontal tail plane. Unlike the Learjet 23s the 24s have a raised central cockpit window frame, increasing its strength. Moreover, the 24 has a sort of handle on the outside in the emergency exit in the second large window on the right, not present at the 23.

The Learjet 24 sub versions mentioned here differ in the engines (though all versions of the General Electric CJ610), maximum weights and internal systems.

These Learjet 24s still have the short fuselage, but the large cabin windows have been replaced by more smaller (three on each side). Furthermore, the bullet fairing disappeared.

To increase the performance of the Learjet 24, Dee Howard developed a series of aerodynamic modifications to existing Learjet 24s. The most visible is the thicker wing with wider chord, near the wing root. Hence the leading edge starts below the second cabin window instead of the third window.

The Learjet 25 is the stretched version of the Learjet 24 and has, in most cases, four small windows on the left side and five on the right. But there may also be less windows! In addition, the first subversion of the 25 also has the bullet fairing of the early model Learjet 24s on the tail.

These are also noisy Learjet types, which differ from the first model Learjet 25 by no longer having a bullet fairing. Between the subversions no differences are visible on the outside.

The performance enhancing measures of the Learjet 24XR are also available for Learjet 25s. The modifications are slightly different, but also include a re-designed inboard wing like the Learjet 35ZR (see below).

Learjet built some of these Learjet 25s as new, under the designation Learjet 25G. This version is not included on Learjet 25 type certificate, but there is only a supplemental type certificate (STC).

In the early eighties Robertson Aircraft developed a series of improvements for the Learjet 25s, which involved the installation of winglets on the tip tanks and replacement of the original CJ610 engines by JT15Ds. To our knowledge, only one aircraft was fitted with winglets (c/n 25-036), but has none of the other planned modifications. Then it became quiet around the Learjet 25TF project and the aircraft was converted back to Learjet 25.. 

Later Spirit Wing tried the same but then with Williams FJ44 engines. One Learjet 25D (c/n 25D-316) was converted.

Learjet developed a new swept wing for the Learjet, which also has winglets. This was first applied to the Learjet 28, which is actually a Learjet 25D with the new Longhorn wing. So watch for winglets instead of tip tanks on a CJ610 powered Learjet. The Learjet 29 is the long-range version of the 28, with only internal differences.

This is a hybrid of the Learjet 35, of which the fuselage, tail and engines were used, and the Learjet 55, that provided the wings. So it has a narrow fuselage, turbofan engines and winglets. Also standard are the two ventral fins - called “delta fins” - behind the engines. However, these are also available from Avcon as a retrofit on the Learjet 35. There are no external differences between the Learjet 31 and 31A.

Raisbeck Engineering produces an aft fuselage locker as a modification to existing Learjet 31 and 35/36s. In a bulge under the rear fuselage more bagage room is created.

The Learjet 35/36, with subversions, has the largest production run of all Learjet models. This model was created by lengthening the fuselage of the Learjet 25, extending the wingspan and replacing the CJ610s by TFE731 turbofans. As with Model 28 Learjet also offered a version with a longer range, the Learjet 36.
Subtle, so not really visible modifications to the wings resulted in the Learjet 35A and 36A respectively.

Notes on military versions:

• The Learjet 35A is designated C‑21A with the USAF.
• The Learjet 36A is called U-36A in Japan.
• C-35, R-35A and VU-35A are the designations in Brazil.
• A Learjet 35A is called Tp104 in Swedish military service.

Raisbeck Engineering offered really distinguishable wing modifications to improve performance, much like Dee Howard did with the old Learjet. The inboard wing is fitted with a 45° swept glove extending out to about a third of the wingspan (larger than the Learjet 24XR/25XR), creating a sort of double-delta. There is also a smaller triangular fillet between the wing trailing edge and the tip tank. This ZR package seems not available anymore.
The company however still has an aft fuselage locker in its portofolio. This creates a bulge under the rear fuselage.

Avcon Industries is manufacturer of aft fuselage delta fins (double ventral fins) to improve directional stability for the Learjet 35/36.

The Learjet 40 is the replacement for the Learjet 31A. It is a Learjet 45 with a meter shorter fuselage. Therefore, the Learjet 40 has six or seven cabin windows instead of eight. The Learjet 40XR is an upgraded version, offering better performance due to the upgrading of the engines to the TFE731-20BR configuration. This upgrade is also available to Learjet 40s.

The first Learjet since the 23, that has been designed completely from scratch, is the Learjet 45. The differences with the other small Learjets are quite subtle, but if you look closely you can see some clear differences in the cockpit windows and the tail. Furthermore, the 45 has most cabin windows of all Learjets: eight. The Learjet 45XR is a subversion with better hot-and-high performance thanks to modified engines. A Learjet 45 can be easily converted to a 45XR.

The Longhorn wing with winglets got its second application on the Learjet 55, which also has a larger diameter fuselage. It can also be distinguished from the smaller Learjets by the “extra” cockpit side window. In comparison with the regular 55 the Learjet 55B, 55ER and 55LR have internal changes such other instruments, enlarged fuel tanks and higher take-off weights.

Like the Learjet 31, the Learjet 55C has large delta fins under the tail. Furthermore, there are no external differences with the other 55 subversions.

Remark: the Brazilian Air Force flies this type as VU-55C.

Until now, the Learjet 60 is the largest of the Learjet series that has been mass-produced. It has a meter longer fuselage than the 55C. Hence, it has one cabin window more, five on the port side and six on the starboard side. Furthermore, the Pratt & Whitney PW305 engines have thrust reversers fairings that extent significantly behind the engine nacelles. That is the best recognition point. The 60XR only has a different avionics package.

The successor of the Learjet 40XR is most easily recognisable by the different type of winglets: they have an angle of about 45° with the wing, less than the original winglets of the Longhorn wing.

Just like the main difference between the Learjet 40XR and Learjet 70 the so-called canted winglets have an angle of about 45° with the wing, less than on the Learjet 45XR.

To attract more customer Bombardier offers a lower cost Learjet 75, marketed as Learjet 75 Liberty. It has six seats and standard no APU.

The banner of largest Learjet was to be taken over by the Learjet 85. Just one prototype had flown before the development programme was cancelled. The Learjet 85 has wings that go underneath the fuselage, therefore not protruding the cabin. To reduce drag it has a large wing-fuselage fairing.