Roberts later wrote: "Shortly after the tragedy of President Kennedy's death, November 22, 1963, Miss Eva Adams, the Director of the Mint, telephoned me at the Philadelphia Mint and explained that serious consideration was being given to placing President Kennedy's portrait on a new design U.S. silver coin and that the quarter dollar, half dollar or the one dollar were under discussion.
"A day or so later, about November 27, Miss Adams called again and informed me that the half dollar had been chosen for the new design, [as] Mrs. Kennedy did not want to replace Washington's portrait on the quarter dollar. Also it had been decided to use the profile portrait that appears on our Mint list medal for President Kennedy and the President's Seal that has been used on the reverse of this and other Mint medals."
Coinresource.com picks up the story from there: "This work was undertaken immediately, Gilroy Roberts sculpting the portrait obverse, while his long-time assistant engraver, Frank Gasparro, prepared the reverse model bearing the presidential seal. Both were amply experienced in these tasks. Along with the sculpting of various mint medals, Roberts had prepared the models of John R. Sinnock's design for the Benjamin Franklin half dollar of 1948, following Sinnock's death the previous year. Gasparro too was a veteran of numerous medal designs, and he had most recently created the new reverse which debuted on the Lincoln cent in 1959. For these two artists, time was of the essence, as the new year loomed ahead, and the Treasury Department did not want to issue any of the existing-type Franklin half dollars dated 1964. Complicating matters still further was a severe, nationwide shortage of all coins. Half dollars of one type or the other had to be ready for coining early in the new year to avert a worsening of this shortage.
"In the meantime, however, there was a legal hurdle to overcome: Under existing law, U. S. coin designs could not be changed more often than every 25 years; the Franklin half was then only 15 years old, and its replacement would quite literally require an act of Congress. Partisan disputes were largely set aside in recognition of the nation's and the world's loss, and Congress managed to pass legislation permitting a change in the half dollar's design with only a few weeks' debate. The Act of December 30, 1963 made the Kennedy half dollar a reality."
And yet the Kennedy half dollar also marks the virtual demise of the 50-cent coin in the United States. In 1964, more than 429.5 million of the coins were minted (both in Philadelphia and Denver), and as recently as the bicentennial coinage of 1975-76, 521 million were minted. The coin went into decline after that. In 2007, only 4.8 million were minted.