Ju 87B-2 was in mass production from December 1939. For the quick conversion of the bomber aimed at use under different conditions, factory modification kits were produced, or Umrüstbasatze. U4 represented skis to be installed on the main and tail landing gears instead of wheels. The Stuka 87B-2 with skis was able to take off and land on snow-covered airfields without slipping into loose snow.