[3][4] Work on the project began at Embraer during the mid-2000s, with early efforts centred around a conceptual derivative of the E190 jetliner of a similar size to the Lockheed C-130 Hercules.

[4] At the 2011 Paris Air Show, Embraer announced plans to launch a stretched version of the aircraft as a civilian freighter.

Major subcontractors in the aircraft's manufacturing include Aero Vodochody, BAE Systems, and Rockwell Collins.

[9] Between 2005 and 2007, it investigated the pairing of the wing and GE CF34 engine of the mature Embraer 190 (E190) with a cabin that was modified to function as a cargo hold, complete with a rear ramp, closed-loop fly-by-wire system, and synthetic vision.

[10] By 2006, Embraer was studying a military tactical transport design of a similar size to the Lockheed C-130 Hercules, to be powered by 17,000–22,000 lbf (75.6–98 kN) jet engines, such as the Pratt & Whitney PW6000 and Rolls-Royce BR715.

Referred to by the company designation C-390, this transport aircraft was said to incorporate many of the technological solutions present on the Embraer E-Jet series and feature a rear ramp for the loading and unloading of a wide range of cargo.

Simultaneously, the Brazilian Air Force was in the process of finalizing an initial purchase contract for between 22 and 30 aircraft, while Embraer was negotiating with possible partners on the programme.

Around this time, the media claimed that the aircraft would be operated by the Brazilian Air Force and the Army and Navy, and that there were unconfirmed sales to other government agencies in the works.

[15][16] At the programme launch, the design was all-new in terms of its fuselage, wing, flight deck, and engine selection.

According to Embraer, the selected jet engine is sufficiently resilient to dust ingestion, whereas propeller tips close to the ground are susceptible to damage.

[3][18] At the 2011 Paris Air Show, Embraer announced plans to launch a stretched version of the C-390 focused on the civil market for freighters sometime around 2018.

To increase internal capacity, two plugs will be added fore and aft of the centre fuselage section, which would also provide a new side cargo door.

[20] In August 2010, the defence ministers of Chile and Brazil signed an agreement for the Chilean aircraft company ENAER to join the C-390 industry team.

[21] That same month, Argentine Defence Minister Nilda Garré announced that Argentina through FAdeA, would participate in the construction programme.

[24] In December 2011, Brazil and Portugal agreed to a defense partnership with Empresa de Engenharia Aeronáutica (EEA) for developing the engineering data for the KC-390's components, which will be manufactured by Embraer's Portuguese subsidiary OGMA.

[28] In November 2019, it was announced that Boeing and Embraer were to form a new joint venture company to promote and develop new markets for the C-390 Millennium.

[9] In February 2021, Embraer and the Brazilian Air Force dispatched a single KC-390 to the US to undergo testing under extreme cold conditions.

[57] In the first quarter of 2023, the Brazilian Air Force fleet of C-390 had accumulated more than 8,000 flight hours, having participated in aviation fairs and carried out missions on all continents including Antarctica, where C-390 from the Fat Squadron (1st/1st GT) performed two air resupply missions for the Brazilian Comandante Ferraz Antarctic Station.

[61] The cockpit has head-up displays for the enhanced vision system with four cameras and Rockwell Collins Pro Line Fusion commercial avionics.

[64] The C-390 has a cruising speed of Mach 0.8 which, according to Embraer, enables payloads to be transported faster than any other airplane in the medium airlift market.

[9] The aircraft is equipped with fully fly-by-wire flight controls combined with active sidesticks, which reduces the crew's workload over conventional counterparts and permits load factors up to 3g.

This allows for the carriage of either two fully-tracked M113 armored personnel carriers, one Boxer or Brazilian VBTP-MR Guarani wheeled armoured vehicle, a Sikorsky H-60 helicopter, 74 litters with life-support equipment, up to 80 soldiers or 66 paratroopers with full gear.

[71] Typical amenities present in the hold include a galley, an accessibility-friendly toilet, automatic temperature control, and noise/vibration mitigation measures.

[74] There was the stated intention of progressively replacing the Brazilian Air Force's existing cargo aircraft fleet with the type, including its C-130s.

[76] The fleet of C-390s will be operated from Anápolis Air Force Base by the 1st Troop Transportation Group (1º GTT) Zeus and in Rio de Janeiro by the 1st/1st GT Gordo.

[78] In December 2024, Embraer and the Brazilian Air Force (FAB) signed an agreement at Mostra BID, the National Defense and Security Fair in Brasilia, to deepen collaborative studies aimed at expanding the capabilities of the C-390 Millennium platform for Intelligence, Surveillance and Reconnaissance (ISR) missions, with a focus on Maritime Patrol.

[94] In November 2020, almost six months after the Hungarian Air Force retired its last Soviet-era An-26 military cargo plane, Hungary signed an order for two KC-390s, along with full training, support and maintenance.

[citation needed] In December 2023, the South Korean Defense Acquisition Program Administration (DAPA) decided to introduce three KC-390s by 2026, with a budget of KRW 710 billion (US$540M).

[112] BAE Systems signed an agreement with Embraer to market the C-390 to Kuwait, Qatar, Oman, Saudi Arabia and United Arab Emirates.

[125] In April 2024, Embraer and Brazilian postal company Correios signed a Memorandum of Understanding focusing on air cargo transport.