Production of the Firefly started in January 1944 and, by 31 May, some 342 Sherman Fireflies had been delivered to Montgomery's 21st Army Group for the D-Day landings. As a result, British tank troops were composed of three standard Shermans and one Firefly. The same distribution occurred in Cromwell units, but this caused logistical problems, as each Cromwell troop then needed to be supplied with parts for two different tanks. The Firefly was also slower than the Cromwell. Churchill units received no Fireflies, and as a result often had to rely on any attached M10 or M10 Achilles units to provide increased firepower to deal with tanks their own guns could not eliminate.