On May 4, 2019, a fully-functioning, full-sized prototype of the Lilium Jet flew for the first time. A 1:2 scale-model made a 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 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.