In 1969 Union Pacific was retiring the later gas turbine-electric locomotives. Union Pacific had ordered EMD DD35s and DD35As to replace the turbines, and the DDA40X was a further development. Forty seven were built between June 1969 and September 1971, except the first one delivered in April in time to participate in the celebrations of the centennial anniversary of the completion of the Transcontinental Railroad driving the "Gold Spike Limited" and arriving in Salt Lake City, Utah, on the morning of May 10, 1969. The units were numbered from 6900 to 6946, with 6936 still in service.
The DDA40X is 98 ft (30 m) long. The frames were fabricated by an outside contractor, the John Mohr Company of Chicago, since the frame length exceeded the abilities of EMD's plant. Using more than one prime mover in a single locomotive was not new; the E-series were popular dual-engine locomotives, and Baldwin had produced (but not sold) a locomotive with four diesel engines.
The 'X' in the designation stood for eXperimental, as the DDA40X locomotives were used as the testbeds for technology that would go into future EMD products. The modular electronic control systems later used on EMD's Dash-2 line of locomotives were first used on the DDA40X. The locomotive was the first to be able to load-test itself using its dynamic braking resistors as an electrical load so that external equipment was not required. The DDA40X used the wide-nosed cab from the FP45cowl units. This design was superficially similar to the Canadian comfort cab introduced by Canadian National soon afterwards in 1973, but it lacked the structural reinforcements introduced in the CN design that were carried over to future wide-nosed cabs.
As the DDA40X program was a test, a number of experiments were conducted during the service life of these locomotives. One such test included fitting a few of the units with air raid sirens to warn track-side personnel when away from grade crossings, but the results were inconclusive. Another of the tests included successful modular electrical components. This made for easier diagnosis of electrical problems. These modifications were used in all future locomotive units built by EMD. All DDA40X units included a new load test circuit, allowing units to load test without a track-side load test box. Gearing was 59:18, allowing 80 mph on passenger trains.
By 1974, averaging 22,000 miles a month, most DDA40X units had run over 1,000,000 miles and needed more maintenance. In the early 1980s, this factor, along with other issues including the higher efficiency of newer locomotives such as the SD40-2 and the national recession led to all 45 remaining units into storage. In early 1984 as rail traffic rebounded, Union Pacific brought 25 DDA40X units out of storage and rebuilt them to return to service; 15 unserviceable units were retired in June that year. All DDA40X units were retired by May 1985. Number 6936 however is still in service with UP, though mostly in excursion service.