These units were very similar to earlier Polish Orzeł class, with a reduced torpedo battery and 10 vertical mine tubes fitted on each beam.

The boats were first planned as K XIX and K XX (K stands for submarines intended for Dutch Colonies) but during the construction it was decided that they were going to be put on the budget of the Department of War, which resulted in the names getting changed to O 19 and O 20.

These minebuns, which were located outside the pressure-resistant double skin, could each accommodate two mines that could be discharged separately from each other hydraulically.

When it comes to sonar the O 19-class submarines were equipped with noise pulses from the German company Atlas Werke, these were precursors of the Gruppenhorchgerät which were used by the famous U-boats during the Second World War.

[4] In 1943, during a major maintenance period in Great Britain, the noise spanner of the HNLMS O 19 was replaced by an Asdic system of type 120B.