The Ultron Sentries are an entire legion of robotic soldiers originally designed by Hank Pym and Howard Stark during the former's tenure at S.H.I.E.L.D. with the intent of serving as prototypes to map the human mind onto an artificial intelligence. However, the robots never got to be constructed as Hank quit S.H.I.E.L.D. some time later due to the organization trying to replicate his prized Ant-Man shrinking technology against his wishes. Nonetheless, while the plans for the Pym Particle were taken away by Hank, the schematics for both the Ultron A.I. and Sentry robot remained within the SHIELD database vaults for many years until Wolfgang von Strucker, one of HYDRA's original commanders, managed to get inside the organization and manipulate several scientists into giving him the plans for the Ultron Sentries with the intent of having them constructed to serve as his future army in the inevitable uprising. It wouldn't be until years later when the Chitauri Scepter and its Mind Stone were recovered from the Battle of New York did Strucker gain the final piece necessary to begin his vile experiments and mass production. However, Strucker only managed to complete work on several of Hank's Sentries, now rebuilt to bear similarities to the Chitauri soldiers as well as the Ant-Man armor, before his defeat at the hands of the Avengers and later capture by NATO. When the Ultron A.I. finally came online and learned about his history, goals, and programmed mission following his retrieval from the databanks by Tony Stark and Bruce Banner, he discovered about the combined efforts from Tony, Howard, Bruce, Hank, and Strucker to work on him and his prototypes, and, feeling inspired by their work, transported his consciousness to Strucker's abandoned headquarters in Sokovia to finish construction on the entirety of the Sentry army, culminating in him building for himself a body similar to the Sentries but more human-like and expressive to show off his inner "inhumanity" and on how he was their true leader.
All of the Sentries were ultimately destroyed in the Battle of Sokovia, much to Hank Pym's secret shame and relief, although their shattered parts were later recovered by Gideon Malick's HYDRA troopers to sell on the black market for a hefty price in their operations. The Sentries were later rebuilt by Ultron himself to serve him again when he entered an alliance with Thanos, the Mad Titan, as a battle commander.