In 1928 Willoch took part in the search for the Latham 47 flying boat that had disappeared over the Barents Sea while carrying the Norwegian explorer Roald Amundsen.

The northern port of Narvik, defended amongst other sea and land units by HNoMS Eidsvold and her sister ship HNoMS Norge, was one of their most important targets due its role as an all-year export port of Swedish iron ore. As part of the Royal Norwegian Navy's efforts to protect and defend Norway's neutrality in the Second World War Norge and Eidsvold, the largest and most heavily armed Norwegian warships in service, had been based in Narvik.

[6] For several weeks the Norwegian coastal defence ships had been preparing for war, carrying out intense gunnery exercises with live ammunition.

[8] Soon after radio messages were received from the patrol boat Kelt further out in the Ofotfjord, relayed from Norge,[9] that nine German destroyers were sailing towards Narvik.

By 0400hrs visibility was so poor that the crew of the Eidsvold could not see land although they were only around 500 metres (550 yd) from shore, at which point Willoch ordered that his ship was to raise anchor.

When the intruding ships failed to reply to the light signals Eidsvold fired a warning shot[11] from one of her 76 mm (3 inch) guns.

[12] In response to the warning shot the German flotilla commander, Kapitän zur See Friedrich Bonte, despatched an emissary to the Eidsvold.

[11] While this was going on the German destroyer Wilhelm Heidkamp had positioned herself 700 metres (770 yd) off the port side of Eidsvold and trained her torpedo launchers on the Norwegian ship.

[11] One of the German torpedoes hit the main ammunition hold, tearing apart Eidsvold, killing many crewmen and leaving the initial survivors swimming in water only just above freezing.

[3] Willoch was awarded with the Haakon VII Coronation Medal, he was a Knight of the Icelandic Order of the Falcon and a Chevalier of the French Légion d'honneur.