The Junkers J.I (manufacturer's name J 4) was a German "J-class" armored sesquiplane of World War I, developed for low-level ground attack, observation and army cooperation.

[2] It was an extremely advanced design for the period, with a single-unit steel "bathtub" running from just behind the propeller to the rear crew position acting as armour, the main fuselage structure and engine mounting in one unit.

The armour was 5 millimetres (0.20 in) thick and weighed 470 kilograms (1,040 lb) and protected the crew, the engine, the fuel tanks and radio equipment.

[3] The aircraft could be separated into its main components: wings, fuselage, undercarriage and tail, to make it easier to transport by rail or road.

The aircraft could be fitted with two downward-firing machine guns for ground attack but they were found to be of limited use because of the difficulty of aiming them.

This aircraft was previously exhibited at the Leonardo da Vinci Museum at Milano and was restored at the Deutsche Technikmuseum Berlin between 2005 and 2010.