The HK45 began life as a now-defunct project to create a Heckler & Koch 1911-style pistol with Larry Vickers, an ex-Delta Force operator, and Ken Hackathorn. Though that project was cancelled due to the market price of such a weapon, their contributions led directly to the HK45.
"The HK45 [was] developed to meet the needs of the most distinguished, elite U.S. military operators." As such, the HK45 was designed to meet requirements set forth in the U.S. Military Joint Combat Pistol program which had the purpose of arming the U.S. Military with a .45 ACP semi-automatic pistol instead of the 9mm M9 pistol. The Joint Combat Pistol program was suspended indefinitely in 2006, and the M9 pistol remains the standard issue handgun for the U.S. Military. Even though the Joint Combat Pistol program had ended, HK decided to make the HK45 available to civilians and law enforcement/military groups to purchase.
The HK45 represents an evolutionary advancement of the Heckler & Koch USP. It is available in the same ten variants as the USP. The HK45 is a full size model pistol, but significant effort went towards making it more ergonomic than the HK USP full-size chambered in .45 ACP by incorporating features found on the Heckler & Koch P30. These include an extended slide release, a textured ergonomic grip with finger grooves, and interchangeable backstraps to fit differently sized hands. In order to accommodate the smaller, more ergonomic grip, the HK45 has a magazine capacity of 10 rounds versus 12 rounds of the USP. The HK45 also added grip serrations on the front end of the slide, a Picatinny rail in front of the trigger guard for mounting accessories, and an O-ring polygonal barrel for increased accuracy.