The heart was added to the Douglas armorial bearing (which previously consisted only of the three stars) following the death of Sir James Douglas in 1330 while carrying the heart of Robert Bruce on crusade (probably at Teba in Spain).

From The heraldry of the Douglases

DOUGLAS ARMS.— The Seal of Sir William Moray, 1296, shows three stars as the charge (Laing). In the Armorial de Gelre (fourteenth century), these Arms appear as azure, three stars argent. The Seal of Sir William Douglas of 1296 shows on a chief three stars, and in the Armorial de Gelre these Arms appear as argent, on a chief azure three stars of the field (with heart in addition). From the similarity of the Arms it is supposed that the Murrays and Douglases had a common origin. The Douglases then branched into two lines — the senior retained the azure chief with three stars ; the junior, that of Morton, changed the chief to gules and reduced the stars to two. The senior line with the blue chief added a red heart to the field in commemoration of the fact that the Good Sir James carried Robert Bruce's heart at least part of the way to the Holy Land. The first Marquis of Douglas added a crown to the heart to show that it was that of a king. The junior line with the red chief also branched out, and the younger branch, that of Lochleven, made the chief dancetty, and the three teeth were later lengthened into piles.