On May 4, 2019, a fully-functioning technology demonstrator version of the Lilium Jet flew for the first time. A 1:2 scale-model made its first flight in 2015 and a larger scale technology demonstrator flew in April 2017. 

On October 22, 2019 the German company announced the completion of the first phase of flight testing. The aircraft achieved speeds exceeding 62 mph (100 km/h) and conducted a variety of maneuvers, including the transition from vertical to horizontal flight. Subsequently, Lilium started further flight testing, but on February 27, 2020, the technology demonstrator was destroyed by fire during maintenance work. The company started to build a second technology demonstrator that it said would incorporate lessons learned from the first model. However, this work was delayed by disruption from the Covid pandemic and, as of September 2020, Lilium would not say when flight testing will resume.

Lilium's business plan calls for it to have the aircraft certified and ready to be operated in several cities around the world from 2025. In June 2019, it announced plans to establish its main software engineering team in London, mainly to deal with the software development requirements for its plans to operate and commercialize on-demand air taxi service.

The company is making use of program management systems devised by Airbus, where some of its senior leaders previously worked. This includes stage-gate review processes to ensure that quality standards are being met at every key point in the program. It is also actively engaging with suppliers to ensure that their various contributions meet the same standards and are on track to meet requirements for certification and production. 

As of September 2020, the company employed 500 people, with most of these engaged in engineering work. The Lilium team includes just over 35 PhDs and more than 40 nationalities. Around 90 of these people have already been assigned to prepare for production ramp-up.

The aircraft is powered by 36 all-electric engines mounted on flaps that provide vertical or horizontal thrust as the wings are tilted. The design, which is unconventional even by eVTOL standards, has no tail, no rudder, no variable pitch, no propellers, no gearboxes, no oil circuits, and only one moving part in each engine. Lilium argues that the reduced number of components makes the aircraft safer and more affordable to operate.

As of June 2020, Lilium had raised a total of more than $375 million in investments. This included around $275 million from a funding round launched in March 2020, which included a further $35 million investment from Baillie Gifford announced on June 9. 

On July 14. 2020, Lilium announced a partnership with Toray Industries, a carbon-fiber manufacturer that promises to supply the eVTOL developer with composite materials for the new Lilium Jet. Known especially for their work with the 787 Dreamliner, Toray materials will be used to build further iterations of the all-electric aircraft.

On September 8, 2020, Lilium announced a partnership with Cologne-Bonn and Dusseldorf airports to explore options for developing an air mobility network for the German state of North Rhine-Westphalia. 

In November of 2020, Lilium announced a deal with the city of Orlando, Florida, to launch its first U.S.-based hub for its operations. Dubbed a "vertiport" but the Munich-based company, the facility would be just outside of Orlando International Airport in a rising community known as Lake Nona. The city offered Lilium more than $800,000 in tax rebates, paving the way for commercial operations to begin by the hub's expected completion in 2025.

On December 9, 2020, the company announced a partnership with Lufthansa Aviation Training to establish a qualification program for prospective Lilium Jet pilots. With eVTOL aircraft and regional air mobility services still in their infancy, Lilium dubbed the training program the first of its kind, and a significant milestone on its path to launch.

On January 5, 2021, the company announced its new chief technology officer Alastair McIntosh, who formerly headed engineering and technology at Rolls-Royce Germany. One week later, former Airbus CEO Tom Enders joined Lilium's executive board. This furthers Lilium's strength and credibility as a serious player in the eVTOL industry as it aims to meet its 2025 service goal.

On February 22, 2021, Lilium selected Aciturri, an aerostructures and aero-engine components production company, to produce the primary composite structures on the Lilium Jet eVTOL. Both Lilium's chief program officer, Yves Yemsi, and Aciturri come from a past of collaboration on projects like the Airbus A350 XWB, and while Lilium says it is putting a lot of responsibility on the the aerostructures company, it remains trustful and prepared.

On March 30, 2021, Lilium announced plans for an $830 million Nasdaq flotation through a merger with special purpose acquisition company Qell. At the same time, the company confirmed that the series production version of the Lilium Jet will have seven seats (with one being for a pilot). It aims to complete concurrent EASA and FAA type certification in 2024.

On September 15, 2021, with its "business combination" with Qell complete, the merged company's shares floated on New York's Nasdaq exchange. The deal is now expected to raise around $584 million in gross proceeds. This is less than the $830 million anticipated when the SPAC merger was announced, which a source close to the company told FutureFlight is due to the fact that 65 percent of Qell shareholders opted to redeem their stock on closing. With financial obligations met, attention will now shift to the speed at which the Lilium team can step up development and certification work for their seven-seat aircraft. Most immediately, it needs to accelerate plans for the flight testing of a production-conforming prototype.

In a February 28, 2022, letter to shareholders, the company disclosed that it is changing the Lilium Jet's design to have six fewer ducted fans: 30 versus 36. The company said that this change has been made possible by using a slightly larger and more powerful engine design, and will reduce the parts count, weight, and system complexity, as well as improve the aerodynamic balance between the main wing and canards. It added that the decision should also result in reduced maintenance costs and support design flexibility for future eVTOL models. In January, the company had announced that it was preparing to resume flight testing with its fifth-generation eVTOL technology demonstrator, which is smaller than the planned production aircraft. 

Lilium reported that its total cash spending for the fiscal year ending on December 31, 2021, was slightly above projections at $217 million and that it expects spending in the first three months of this year to be somewhat below earlier projections. The company said that, based on ongoing evaluations of its program and launch timeline, it will provide an update on the next phase of the program after the end of the first quarter.

February 2022 also brought news that private aviation group NetJets intends to add 150 Lilium Jets to its fractional ownership fleet and market these to its clients. The U.S.-based group is part of Warren Buffett's Berkshire Hathaway investment group and could act as an operating partner to support Lilium's planned network of shuttle flights. NetJets' sister company FlightSafety International will provide flight training for eVTOL pilots.

In late March 2022, Lilium was permitted to continue flight testing in Spain following the loss of its first prototype in a February 2020 fire. On March 31, 2022 Lilium announced it was delaying the projected timeline of its eVTOL by a year or more, pushing its projected type certification into 2025.

In April 2022, Lilium marked an important step in the type certification process for its eVTOL design as it submits a complete set of means of compliance proposals to EASA. The EASA will consider if Lilium’s eVTOL meets airworthiness requirements.

On May 30, 2022, Aernnova agreed to produce propulsion flaps for Lilium’s eVTOL design. The following day, on May 31 2022, Lilium made a breakthrough as it stated it has achieved the critical transition from vertical hover to wing-borne cruise flight with its eVTOL design, the first of said transition for a full-sized electrical aircraft.

In June 2022, Lilium partnered with Expliseat to produce seats 35% lighter than the typical aluminum aircraft seat for the Lilium Jet.

In July 2022, Lilium became the fourth in a series of eVTOL developers including Vertical Aerospace, Eve, and Overair to have a partnership with helicopter operator Bristow. Under a non-biding agreement, Bristow will provide maintenance for Lilium eVTOL's scheduled flights in Florida.

In February 2023, Collins Aerospace became the latest partner for the program through an agreement to develop the interceptor sidestick controls for the Lilium Jet. Other key suppliers already include Honeywell (avionics, flight control computer and electric motors), Denso (electric motors), CustomCells (batteries), GKN Aerospace (electrical wiring interconnection system), Astronics (power distribution systems), ABB E-mobility (recharging systems), Aciturri (aerostructures) and Toray (composite materials).