The Model 45 was developed to make Learjets fundamentally more competitive against newer designs from competing manufacturers.

FAA certification was delayed, and finally granted in September 1997, with the first customer aircraft subsequently delivered in mid-1998.

[2][3] The aviation magazine Flying reported that the Lear 45 was first certified under FAR Part 25 (transport category rules) in 1998.

[5] Problems came to a head in August 2003, when the FAA discovered a defective fastener for the Model 45's horizontal stabilizer could break, sending the airplane into a fatal dive.

[5][8] However, 10 years later, a 2017 "Used Aircraft Report" by Business & Commercial Aviation magazine indicated that owners now regard the aircraft as "gas-and-go airplane" and credit it "with great reliability," and the article's author, B/CA senior editor Fred George, describes it as "a rock-solid reliable workhorse.

)[6] Previous Learjet designs had used rigorous, extensive "fail-safe" structures in the wing and tail assemblies, with numerous wing spars providing a highly reinforced structure (derived from the Swiss-designed P-16 prototype fighter that was the original basis for the first Learjet)[10] – but the Model 45, instead, used a lighter, less-robust structure, affording less cost in manufacture and lower aircraft empty weights, resulting in improved capacity, efficiency and some performance enhancement (for a given amount of engine power), at a competitive price.

[4] It also incorporated two conspicuous modifications pioneered on earlier Learjets: the "delta fins" (twin ventral fins, positioned to help stabilize the aircraft in flight, and help right the aircraft in a deep aerodynamic stall) – and winglets (upturned wingtips to reduce induced drag and improve stability).

[4][11][5][6] At normal atmospheric conditions, zero-wind takeoff runway requirements range from 4,200 feet (for a stripped, basic plane with a basic operating weight of 11,700 pounds) to 5,040 feet for a fully equipped model, with auxiliary power unit and other upgrades (14,200 lbs).

[6] However, the engine exhibits poor performance in high-density altitude situations (such as takeoff from hot, humid or high-elevation airports), often resulting in long runway requirements.

Though there are no leading-edge slats or wing fences, the wing has vortilons on the leading edge, to avoid spanwise flow, and small metal triangles on the leading edge to minimize airflow separation during flight at a high angle of attack.

[4][5] Dual-wheel, trailing link, main landing gear is provided, with digital nosewheel steering facilitating tight turns.

[4][5][6] The XR allowed a gross weight 1,000 pounds greater than the original Model 45, greatly increasing payload and fuel/range options.

[4][6][12] The Model 45 cockpit is equipped with a four-screen Honeywell Primus 1000 EFIS avionics system,[6] with TCAS and EGPWS.

However, the aircraft does come equipped with an auxiliary power unit, and thrust reversers (rare in light business jets).