Here is the only info on this topic:
In 1945 as part of the preparation for Operation Downfall (the planned invasion of Japan) a proposal was discussed in the War Department to appoint Douglas MacArthur to the rank of General of the Armies. Following the use of the atomic bomb in August 1945 and the subsequent Japanese surrender, the proposal was dropped.
The matter was raised again in 1955, when the United States Congress considered a bill authorizing President Dwight D. Eisenhower to promote MacArthur to General of the Armies, in recognition of his many years of service. At that time, the Army Judge Advocate General warned that, should MacArthur accept promotion to the new rank, he would lose a large amount of retirement pay and benefits associated with the much more firmly established rank of five-star General of the Army, which he still held. The Army General Staff was also concerned because George C. Marshall was senior to MacArthur and that, should MacArthur be made a General of the Armies, a similar measure would have to be passed promoting Marshall as well. Because of the various complications, MacArthur declined promotion and the bill to promote him was dropped.
But some people continued to push for MacArthur to be promoted. The MacArthur Memorial in Norfolk has numerous letters on file dating from 1962 to 1964 between advocates (former MacArthur aides and others) and government officials attempting to obtain the six-star promotion. In the letters, in a congressional record appendix from February 1962 (pages A864-A865), and in the bill to promote him, this promotion was referred to alternately as "six-star general" and "general of the armies." The proponents even obtained a vote of neutral support from Harry Truman. (He would neither support nor attempt to scuttle the promotion.) The proponents' promotion attempts were ultimately scuttled by the John F. Kennedy assassination and then MacArthur's death in 1964.
