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Origin of the Flag of Siam
The national flag of Ancient Siam is unknown, however, there were some evidences that in the reign of King Narai (1590-1605) the red flag was hoisted to salute the French ship. The red flag was also used in the reign of King Boromokot (1732-1758) when Thai monks were sent to Sri Lanka. Thus, Prince Damrong Rajanub hab concluded that the red flag was the national flag of Siam in the ancient time. Moreover, a plain red flag were used when Siamese armies went into battle to distinguish them from opposing forces. This was also the case for trading boats when they went to other kingdoms; during the Ayutthaya period, for example a plain red flag was used by Siamese merchant boats. 
The red flag was used as the national flag in the Ayudhya period and continued to the Thonburi period. Until the reign of King Rama I, the first King in the Rattanakosin period, the King changed the national flag by adding a wheel (Chak) in the middle of the red flag (1782 - 1809) and used it to symbolize the royal ships. 
Early in the Rattanakosin (Bangkok) era, a trading post was established by the British in Singapore, and two royal ships built by King Rama II , displaying the red flag, went there and to Macao to trade. Eventually the English governer general of Singapore sent an officer informing the King that commercial boats from Malaya displayed a similar flag and asking him if another could be used so that royal ships could be distinguished and received in a proper manner. As it happened. In the reign of King Rama II, three white elephants were offered to the King, which was regarded as the special event. Thus, the King declared to charge the national flag with a white elephant (Chang) in the wheel (Chak). This flag was only used by the royal ships (symbolizing the King Who Owned White Elephants) when they sailed to trade with other countries (1809 - 1851) while the plain red flag was still used by the private ships. 
The national flag was changed again in the reign of King Rama IV. The King viewed that the plain red flag used by the private ships cannot distinguish Thai ships from foreign ships. So the King decided to take the wheel out (The 
wheel or called Chak must be used for King only) and keep only the white elephant remained on the red flag (After the wheel took out, the size of white elephant was changed to be bigger). This national flag was used by both royal and private ships (1851 - 1916). 
The red flag with a white elephant was the national flag since the reign of King Rama IV until the reign of King Rama VI. However, the national flag was slightly changed by King Rama VI in 1916. At this time, the white elephant stood on a stand and overdressed instead (1916 - 1917). 
In 1917, the national flag was again changed. King Rama VI considered that his people cannot afford the elephant flag since it was made in foreign countries. So they mostly used white and red cloth instead of the national flag. Occasionally, the national flag with a white elephant was flown upsidedown by mistake. To solve the problem, the King declared to use the red flag with horizontal white stripes as the national flag. 
The red flag with white stripes was used as the national flag in 1917 and was changed its design in the same year. King Rama VI, who originally used the red and white striped flag as the national flag, was suggested by a columnist of the Bangkok Daily Mail Newspaper that a central stripe of red should be changed to blue. The reasons for the changing were that blue was the colour of the King and the red white blue coloured flag, which was similar to the national flags of alliances, would remind the participation of Thailand in the World War I. The King agreed with this idea and declared the Flag Act B.E.2460 (1917) stating that the red white blue coloured flag, which was known as Trairanga Flag, was the national flag of Thailand. Source : www.siamflag.org