(thanks to Steve Chung for his review of Action Comics #399, "Superman, 
You're Dead... Dead... Dead!", which is quoted liberally below)

(Lenny Carlson notes Earth 399 doesn't have jumbo jets and airplanes 
outside of WWII era... it's possible that they might still exist but
travel by blimp is evidently more popular, as portrayed in Animal Man)



1770's - 1780's
====================================
 George Washington pens the Emancipation Proclamation, which frees
the slaves in the American colonies.  An important moral document,
it soon results in the ending of slavery throughout the British 
Commonwealth.  [speculation]

1860's
============================
 Abraham Lincoln leads the country in the Revolutionary War, 
achieving independence from Britain.  He is elected President for 
life.  [speculation]

1870's
============================
 Lieutenant Colonel George Armstrong Custer achieves a position 
of honor among the various native tribes and is elected Chief of 
the Indian Federation.  [speculation]

1940
============================
 The man known as the Unknown Soldier has first case.  [Unknown Soldier #249]
After his death he is buried in Arlington Cemetary.

1945 - 1946
============================
 At the Nuremburg trials, Adolph Hitler is found guilty of war crimes
and is sentenced to death by hanging.

1954
============================
 Clark Kent/Kal-El begins his career as Superman. 

1955
============================ 
 Bruce Wayne begins his career as Batman.

1960's
============================
 An energy vampire drains Superman's powers... leaving the man of 
steel to die.  When he is found on a deserted beach, Superman is 
brought to the National Research Institute, where scientists keep 
the hero alive... until the remnants of his super-abilities can be 
increased and transferred into a bio-duplicate of the original 
Superman
 Superman II visits the leper colony of Riggor... where people with 
incurable space diseases are quarantined...  A week later, Superman II
is at the research institute... his body dissolving into primitive
protoplasm by a strange disease.  The super-brain with control of
its super-powers is still intact and tape records of the first man 
of steel's cytoplasmic structure are also available.  Now it is 
Superman III who leaves the institute... but the scientists know 
that the artificial protoplasm they used to create him is unstable.
Nevertheless, Superman III establishes himself as the most courageous
of them all.

1963
===========================
 President John F. Kennedy pretends to be Clark Kent in order to
prevent Superman's secret identity from being revealed. Later that
year, Kennedy is assassinated by Sirhan Sirhan after midnight. After
his death he is buried in Arlington Cemetary. [Action #309; speculation
by Lenny Carlson]

1965
============================
 Buddy Baker has adventures as Animal Man.  [Strange Adventures #180]

1967
============================
 Rupert "Ken" Kenboya, a native African, has adventures as B'wana Beast.  [Showcase #66]

June 1969
============================
 37 year old Edward Kennedy along with his secretary drowns in Chappaquiddick,
Massachusetts during their car accident. 

1971
============================
 WGBS roving reporter Clark Kent hears a bulletin in his mobile news
van, and Superman is soon heading for Lookout Mountain, where the 
solar furnace is out of control.  He reaches the area at super-speed.... 
where he is told by scientists that the experimental solar power 
generator has exploded... and the chain reaction will ignite the 
atmosphere... turning the planet into a ball of fire.  Superman III 
carries the solar generator into space... where it will be harmless.  
He meets his end when he saves the planet from the explosion caused 
by the new type of energy, which dissolves his unstable protoplasm.

1981
===========================
 American boxer Joe Louis accidently assassinated by Mark Chapman
during his first meeting with musician John Lennon. After his death
he is buried in Arlington Cemetary. [speculation by Lenny Carlson, 
who notes that Joe Louis possibly never met Lennon within Earth-Prime 
continuity]

1980's
============================
 Marvin Gaye survives a shooting incident and continues his musical
career.  [Animal Man #32]

24th Century
============================
 The Historical Foundation inadvertently brings the Earth-1 Superman 
from April 1971, thinking it is Superman III.  They also bring forth
Washington, Lincoln, and Custer, although it is unknown whether they
are the Earth-1 or Earth-399 versions.