The early success of its Hudsons in operational service with the RAF induced Lockheed to initiate the design of a more advanced version. This was based on the company's somewhat larger Model 18 Lodestar civil airliner, but the resulting aircraft was very similar in appearance to the Hudson.

Designated Ventura I, the new aircraft differed from the Hudson in having a wider and deeper fuselage with a length increase of 16 per cent, more powerful Pratt & Whitney engines each rated at 2,000 hp (1,491 kW), the introduction of a proper ventral gun position with two 0.303-in (7.7-mm) machine guns and, because of the more voluminous fuselage and greater engine power, a bomb bay able to accommodate a maximum bomb load of 2,500 lb (1,134 kg).

When the Ventura was included in the Lend-Lease program it was procured initially for the RAF under the designation B-34, but with the cancellation of British orders the outstanding aircraft began to enter USAAF service initially as R-37s, based on the manufacturer's model number, and later as B-34A Lexingtons, and these were used mainly for maritime patrol. In addition, a small number were procured as navigational trainers, and these were allocated the designation B-34B. In 1942 the US Army ordered 550 examples of a new version with 1,700-hp (1,268-kW) Wright R-2600-31 Cyclone 14 engines, for use in a patrol/reconnaissance role under the designation O-56, but only 18 of these aircraft had been received, becoming instead B-37, before the balance of 532 on order were cancelled.