The flag of Norway is red with an indigo blue Scandinavian cross outlined in white that extends to the edges of the flag; the vertical part of the cross is shifted to the hoist side in the style of the Dannebrog, the flag of Denmark.
It is difficult to establish what the earliest flag of Norway looked like. During ancient times countries did not fly flags. Kings and other rulers flew flags, especially in battle. Saint Olav used a serpent within a white mark at the Battle of Nesjar. Prior to this the raven or dragon was used. Magnus the Good used the same mark as Saint Olav. Harald Hardråde used the raven banner. This flag was flown by various Viking chieftains and other Scandinavian rulers during the 9th, 10th and 11th centuries CE. Inge used a red lion on gold. Sverre used an eagle in gold and red. The earliest known flag which could be described as a national flag of Norway is the one used today as the Royal Standard. Eirik Magnusson used a flag described as a golden lion with axe and crown on red from 1280 and this was since regularly the flag of Norway and of the King of Norway.
The flag is based on the Coat of Arms and was originally only a flag for the ruler of Norway (as it is today). It was later also used on ships and on fortresses until it was gradually phased out during the 17th and 18th centuries. Its earliest certain depiction is on the seal of duchess Ingebjørg in 1318. Around 1500 it became the custom for ships to fly the flag of their home country to identify their nationality. A red flag with the golden lion and silver halberd is depicted as the flag of Norway in a Dutch flag book from 1669-70. At least as late as 1698 the lion banner was flown over Akershus Fortress. The “Norwegian lion” was placed in the colours of all the Norwegian regiments in 1641. In 1748 a decree stated that the Dannebrog should be the only legal merchant flag. Wikipedia