William O. Darby (February 8, 1911 – April 30, 1945) was a career United States Army officer who fought in World War II, where he was killed in action at age 34 in Italy.

[3][8][4][6] Darby was awarded the Silver Star for his actions in Tunisia on February 12, 1943:[8] Without regard for his personal safety, the day previous to a raid, he reconnoitred enemy positions and planned the attack which he led the following morning.

The thorough organization and successful attack led by Lt. Col. Darby revealed his initiative, courage, and devotion to duty which is a credit to the Armed Forces of the United States.Darby was subsequently awarded the Distinguished Service Cross for his actions on March 21–25.

Always conspicuously at the head of his troops, he personally led assaults against the enemy line in the face of heavy machine gun and artillery fire, establishing the fury of the Ranger attack by his skilful employment of hand grenades in close quarter fighting.

Darby received an oak leaf cluster to his Distinguished Service Cross for extraordinary heroism in July 1943 in Sicily:[8] Lt. Col. Darby, with the use of one 37mm gun, which he personally manned, managed not only to repulse an enemy attack but succeeded with this weapon in destroying one tank, while two others were accounted for by well-directed hand grenade fire.Darby took part in the Allied invasion of the Italian mainland in September 1943, and was promoted to full colonel on December 11.

The three existing Ranger battalions were effectively wiped out - killed or captured - in the disastrous Battle of Cisterna during the Anzio campaign in early February 1944, after which they were disbanded.

"Task Force Darby" spearheaded the breakout of the Fifth United States Army from the Po River valley bridgehead during the Spring 1945 offensive in Italy and reached Torbole at the head of Lake Garda.