Federal Communications Commission
A Brief History
Underlined Radex references are hyperlinks

Beginning in 1912, radio was regulated by the Bureau of Navigation of the Department of Commerce. Then, In December, 1921 the Commerce Department formally established a broadcast service, with 360 meters (833 kilohertz) set aside for entertainment broadcasts, and 485 meters (619 khz) designated for official government market and weather reports. The single entertainment wavelength meant that stations were supposed to negotiate timesharing agreements, to keep from interfering with each other.

In late September of 1922 a second entertainment wavelength of 400 meters (750 kHz) was assigned for better quality, higher powered stations. Stations on the new wavelength were designated "Class B" outlets, while those on 360 became known as "Class A" stations. About thirty stations nationwide would eventually qualify to use 400 meters.

On May 15, 1923 the broadcasting service was greatly expanded, with the designation of a band of frequencies, in 10 kilohertz steps, from 550 to 1350 kilohertz. 550 to 1040 were set aside for Class B stations. Class A stations were assigned to frequencies from 1050 to 1350 khz, although existing stations were permitted to stay at 360 meters, as "Class C" stations. In November, 1924 the upper end of the broadcast band was extended from 1350 to 1500 khz, providing 15 additional Class A frequencies.

Until mid-1922 new broadcast stations received three-letter calls. After that, the Commerce Department generally assigned Eastern stations four-letter calls with first "A" and then "B" as the third letter. Later stations generally received specially requested calls, normally four-letter, although a few,  got three-letter ones.

1926 -- Commerce Secretary Herbert Hoover knew his authority to regulate broadcasting was shaky under the 1912 Act, but despite his pleas Congress never acted to strengthen his powers. Finally, adverse legal opinions stripped away his regulatory authority. Scores of new stations took to the airwaves or jumped to frequencies of their own choosing. Chaos was reported nationwide. The
Radex for October 1926 shows what the state of the band was like; but it would not be until the full reallocation of stations 18 months later that some order came to the AM band.

APRIL 27, 1927 -- The Federal Radio Commission, newly created by Congress to straighten out the broadcasting mess, began a year-and-a-half long process to reassign stations to non-interfering dial positions.
Radex from 1927 shows some initial reordering of the band.

NOVEMBER 11, 1928 -- The FRC finally implemented a nationwide reassignment of station frequencies, with stations now classified as Local, Regional, and Clear Channel. See the
Radex for October 1928
which shows change in frequencies by FRC in 1928 before and after, November 1928 and December 1928.

MARCH 29, 1941 - In conjunction with the expansion of the broadcast band to 1600 khz, a major frequency reallocation was implemented nationwide. This change was a coordinated effort of the US, Canada, Mexico, Cuba and several other Caribbean nations.

You can see the impact of NARBA by looking first at Radex March April 1940 which has  a chart of channel changes planned. NARBA, delayed for a year, was implemented in March of 1942 and seen post-implementation in Radex January 1942. The comparisons between the two issues show how the present day AM band was determined.

(Thanks to 
http://earlyradiohistory.us/hist-dc.htm for
the dates and data. Please see that interesting site, too.)