Approved on 18 February 1942, the 31st Engineer Battalion Distinctive Unit Insignia, often called a unit crest or DUI, commemorates the Battalion’s World War II service in the European Theater. Two stars bookending a spade are a reference to the French town of Bischwiller where the unit first suffered casualties in the Ardennes-Alsace campaign. Central Europe, a region where the unit erected bridges, is represented by a wavy chevron. The flower on the spade is edelweiss, an allusion to the Rhineland; the spade and edelweiss, along with the two stars, denote the three battle honors the unit was awarded during World War II. DEMONSTRAMUS, the unit motto, is Latin for “We Demonstrate.”
The Distinctive Unit Insignia is worn by all Soldiers (except General Officers) in units that have been authorized to be issued the device. It is worn centered on the shoulder loops of the Army Green Service Uniform (AGSU) and the blue Army Service Uniform (ASU, Enlisted only) with the base of the insignia toward the outside shoulder seam. Full guidance on wear of the DUI is found in DA Pamphlet 670-1,
Section 21-22, "Distinctive unit insignia" and 21–3(d) and (e),
"Beret" and
"Garrison Cap," respectively.
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The 31st Engineer Battalion was originally constituted and activated as an Engineer Company with the same numerical designation on 1 July 1940 at Fort Belvoir, Virginia. It was designated a Battalion on 15 December 1941 and on 29 April 1942 was redesignated as the 31st Engineers; it would become the 31st Engineer Combat Regiment on 1 August 1942, the designation under which it would earn three campaign streamers (Ardennes-Alsace, Rhineland, Central Europe).
During the Vietnam War, the Battalion fought in eleven of the seventeen named campaigns, garnering a Valorous Unit Award, a Meritorious Unit Commendation, and a Republic of Vietnam Civil Action Honor Medal, First Class. It returned to Fort Lewis, Washington after four years of service and was inactivated 12 March 1972. When it was reactivated in 1986, it was transferred to U.S. Army Training and Doctrine Command where it became a training unit; it would go on to be inactivated two more times (October 1993 and October 2002) before its last activation in 2005. As of Summer 2023, it is still serving as a training unit at Fort Leonard Wood in Missouri.