The Delta IV had two main versions which allowed the family to cover a range of payload sizes and masses: the Medium (which had four configurations) and Heavy.

The latest evolutionary development of the Delta rocket family, Delta IV was introduced to meet the requirements of the United States Air Force's (USAF) Evolved Expendable Launch Vehicle (EELV, now National Security Space Launch (NSSL)) program.

However, the Delta IV entered the space launch market when global capacity was already much higher than demand.

In 2003, Boeing pulled the Delta IV from the commercial market, citing low demand and high costs.

[13] As of 2009, the USAF funded Delta IV EELV engineering, integration, and infrastructure work through contracts with Boeing Launch Services (BLS).

On 8 August 2008, the USAF Space and Missile Systems Center increased the "cost plus award fee" contract with BLS for US$1.656 billion to extend the period of performance through the 30 September 2008 (FY09).

[14] In February 2010, naturalized citizen Dongfan Chung, an engineer working with Boeing, was the first person convicted under the Economic Espionage Act of 1996.

Chung passed on classified information on designs including the Delta IV rocket to China and was sentenced to 15 years.

[17] The possibility of a higher performance Delta IV was first proposed in a 2006 RAND Corporation study of national security launch requirements out to 2020.

A single National Reconnaissance Office (NRO) payload required an increase in the lift capability of the Delta IV Heavy.

All following launches used the RS-68A,[21] and the engine's higher thrust allowed the use of a single standardized CBC design for all Delta IV Medium and M+ versions.

This upgrade reduced cost and increased flexibility, since any standardized CBC could be configured for zero, two, or four solid-propellant rocket boosters.

[2] Possible future upgrades for the Delta IV included adding extra strap-on solid motors, higher-thrust main engines, lighter materials, higher-thrust second stages, more (up to eight) strap-on CBCs, and a cryogenic propellant cross feed from strap on boosters to the common core.

[26] At one point, NASA planned to use Delta IV or Atlas V to launch the proposed Orbital Space Plane,[27] which eventually became the Crew Exploration Vehicle and then the Orion.

According to Aviation Week & Space Technology the study, "found that a Delta IV heavy [...] could meet NASA's requirements for getting humans to low Earth orbit".

[32] The Atlas V is expected to stay in service for a few years after Vulcan's inaugural launch, and the Delta IV Heavy was discontinued in April 2024.

[40] On the Heavy, the main CBC's engine throttles down to 58% rated thrust around 50 seconds after liftoff, while the strap-on CBCs remain at 102%.

Along the back of the CBC is a cable tunnel to hold electrical and signal lines, and a feedline to carry the liquid oxygen to the RS-68 from the tank.

Delta IV CBCs and second stages to be mated and tested in the HIF before they are moved to the pad.

[45] The basic launchpad structure includes a flame trench to direct the engine plume away from the rocket, lightning protection, and propellant storage.

A crane at the top of the MST lifts the encapsulated payload to the vehicle and also attached the GEM 60 solid motors for Delta IV Medium launches.

These arms carry telemetry signals, electrical power, hydraulic fluid, environmental control air flow, and other support functions to the vehicle through umbilical lines.

For Delta IV Heavy launches, the port and starboard strap-on CBCs are also mated in the HIF.

After further testing, the payload (which has already been enclosed in its fairing) is transported to the pad, hoisted into the MST by a crane, and attached to the vehicle.

The strap-on CBCs and then core CBC engines shut down prematurely, even though sufficient propellant remained to continue the burn as scheduled.

[92] On 4 October 2012, a Delta IV M+ (4,2) experienced an anomaly in the upper stage's RL10B-2 engine which resulted in lower than expected thrust.

While the vehicle had sufficient fuel margins to successfully place the payload, a GPS Block IIF satellite USA-239, into its targeted orbit, investigation into the glitch delayed subsequent Delta IV launches and the next Atlas V launch (AV-034) due to commonality between the engines used on both vehicles' upper stages.

[93] By December 2012, ULA had determined the cause of the anomaly to be a fuel leak (into the combustion chamber[94]), and Delta IV launches resumed in May 2013.

[98] On August 12, 2018, another Delta IV Heavy launched the Parker Solar Probe on a mission to explore or "touch" the outer corona of the Sun.

[99] The second GPS Block III satellite was launched with the final Delta IV Medium+ (4,2) configuration rocket on 22 August 2019.