12/31/2011. A biplane bomber series that was to span a decade began in 1923, with an order for the Huff-Daland XLB-1 (s/n 23-1250), a normal three-place bomber that differed from convention only in being fitted with tapered wings and a single 800 hp Packard 1A-2540 engine in the nose instead of the traditional pair of lower-powered engines, one on each side of the fuselage between the wings. Bombs were carried internally, and aiming was done through a window in the belly instead of from the normal nose sighting station. Nine service test LB-1s (s/n 26-377 to 26-385) were identical except for installation of an improved 2A-2540 engine.

This single-engine configuration proved to be unsuitable, and a twin-engined XLB-3 (s/n 27-333) was ordered. This was essentially an LB-1 airframe powered with experimental air-cooled and inverted Liberty engines, one on each side. This power plant installation was abandoned, however, and the same aircraft became the XLB-3A, with 410 hp Pratt & Whitney Wasp
R-1340-1 engines. Crew was increased to five, with conventional nose gunnery and bombing stations.

By the time the XLB-3A was delivered, the Huff-Daland and Co. Inc. at Ogdensburg, New York had been reorganized as Keystone Aircraft Corp. at Bristol, Pennsylvania, so the aircraft was known as a Keystone. The XLB-5 (s/n 26-208), similar to the XLB-3A except for conventional 420 hp Liberty engines, and 10 production LB-5s (s/n 27-335 to 27-344) were delivered before the company change as Huff-Dalands.

Twenty-five LB-5As (s/n 28-001 to 28-025), however, with twin tails instead of the single large rudder with two smaller rudders on each side as used on the LB-5s, were delivered as Keystones, as were all succeeding variants of the LB family.

A major change took place with the appearance of the XLB-6 (s/n 27-344), which was the tenth LB-5 fitted with entirely new straight-chord wings with 525 hp Wright R-1750-1 Cyclone engines suspended between the wings instead of resting on the lower. The 17 production LB-6s (s/n 29-011 to 29-027) were the same except for minor refinements and a revised angular shape to the twin rudders.

The LB-5s and LB-6s entered service with squadrons of the 2nd Bomb Group, which was the sole US-based bombing unit of the USAAC until 1928. Other aircraft of the same type were used by the 5th Group (Composite) in Hawaii. They were supplemented by 18 LB-7s (s/n 28-388 to 28-395, 29-001 to
29-010), produced by Keystone in 1928-1929 with Pratt & Whitney R-1690-3 Hornet engines but otherwise identical with the LB-6s.

Changes of powerplant and some variation in tail shape were wholly responsible for a number of different designations applied to the Keystones in this period. The final two LB-7s on the 1929 contract, for example, were completed as the LB-8 (s/n 29-9) and the LB-9 (s/n 29-10) when fitted, respectively, with geared 550 hp Pratt & Whitney R-1860-3 and 575 hp Wright R-1750 Cyclone engines.

The last two LB-6s were similarly completed with experimental engine installations, 525 hp R-1750-1s in the LB-10 and R-1750-3s in the LB-11. The LB-10 also had a single fin and rudder in place of the twin units which Keystone had standardized in the LB-5As. The LB-11 was converted to
LB-11A when fitted with geared GIR-1750 Cyclones and a single LB-12 was similar to the LB-7.

In 1930, the USAAC abandoned its separate designation categories for light (LB) and heavy (HB) bombers and introduced a single bomber (B) category. At that time, production contracts had been placed for a further 73 Keystone bombers as LB-10A, LB-13 and LB-14, but all were redesignated for delivery in the B category.

The 63 LB-10As appeared as B-3As (s/n 30-281 to 30-343), with the same single fin as the LB-10, and 525 hp R-1690-3 Hornet engines. Of seven
LB-13s ordered, five were completed as Y1B-4s (s/n 30-344 to 30-348) with
R-1860-7 engines and the other two as Y1B-6 (s/n 30-349, 30-350) with
R-1820-1 engines; three more Y1B-6s (s/n 30-351 to 30-353) were converted from B-3As.

The three LB-14s were redesignated Y1B-5 before delivery, with R-1750-3 engines, but it is uncertain whether they were ever completed. The B-3, B-4 and B-5 versions all had single fins and rudders, and production quantities of each were ordered; 25 B-4As (32-117 to 32-141), 27 B-5As (converted from
B-3As) and 39 B-6As (s/n 32-142 to 32-180), with deliveries concluding in 1932. They flew with units of the 7th and 19th Bomb Groups, formed in 1928, in addition to the original 2nd Bomb Group and units overseas.