If you go to the Wiki article linked above, you can find the whole story of how multiple vows of fealty have sprung up over the years under the Origins section, but the last bit on Francis Bellamy is the important one as that's the one used today.

Some useful highlights from that section:

The Bellamy "Pledge of Allegiance" was first published in the September 8, 1892, issue of The Youth's Companion as part of the National Public-School Celebration of Columbus Day, a celebration of the 400th anniversary of Christopher Columbus's arrival in the Americas. The event was conceived and promoted by James B. Upham, a marketer for the magazine, as a campaign to instill the idea of American nationalism in students and to encourage children to raise flags above their schools.[28] According to author Margarette S. Miller, this campaign was in line both with Upham's patriotic vision as well as with his commercial interest.

Francis Bellamy and Upham had lined up the National Education Association to support the Youth's Companion as a sponsor of the Columbus Day observance and the use in that observance of the American flag. By June 29, 1892, Bellamy and Upham had arranged for Congress and President Benjamin Harrison to announce a proclamation making the public school flag ceremony the center of the Columbus Day celebrations. This arrangement was formalized when Harrison issued Presidential Proclamation 335. Subsequently, the Pledge was first used in public schools on October 12, 1892, during Columbus Day observances organized to coincide with the opening of the World's Columbian Exposition (the Chicago World's Fair), Illinois.

James Upham "felt that a flag should be on every schoolhouse,"[27] so his publication "fostered a plan of selling flags to schools through the children themselves at cost, which was so successful that 25,000 schools acquired flags in the first year (1892–93).