By Doug Cameron 

Lockheed Martin Corp. said Tuesday that it expects two-thirds of its 2015 order haul to come in the second half of the year as it pursues contracts to build a new Air Force bomber and an Army truck, two programs that together are worth more than $100 billion.

The world's largest defense contractor by sales also pledged to accelerate its share-buyback program in the second quarter as it reported forecast-beating profits and raised its full-year guidance.

Lockheed Martin Chief Executive Marillyn Hewson said she remained confident of growing domestic and international sales, but its two big Pentagon awards expected during the summer that are focusing investor attention.

The company is teamed with Boeing Co.----which reports Wednesday-- on a bid to build a new Long-Range Strike Bomber for the Air Force, in competition with Northrop Grumman Corp. on a program that is expected to cost more than $80 billion.

Lockheed also faces twin competition from Oshkosh Corp. and AM General LLC to build trucks for the Army to replace the Humvee model, a program that could cost around $30 billion.

"We're going to be significantly back-end loaded in our orders for the year," Chief Financial Officer Bruce Tanner said on an investor call, referencing the potential bomber and truck awards, as well as export deals for missile-defense systems and F-35 fighter jets.

Lockheed ended the March quarter with a $76.9 billion backlog, down 4.4% from Dec. 31, but it left its full-year order and sales guidance unchanged.

The F-35 program remains Lockheed's largest growth driver, and Ms. Hewson reiterated that half the orders for the jet were expected to come from overseas customers over the next few years. Analysts have been cautious on the outlook, flagging possible changes following general elections in the U.K.--an existing customer--and Canada, which has deferred a decision on buying new fighter planes.

Lockheed's backlog and rising margins have helped the company outperform most U.S. defense peers over the past two years, alongside the sector's largest stock-buyback program. The company spent more than $600 million on stock buybacks in the quarter, and Mr. Tanner said this could rise to $1 billion in the current quarter, though the company retained its guidance for at least $2 billion in repurchases for the year.

The company made no mention of potential acquisitions, having spent around $900 million last year on a series of small deals in space, cyber and commercial aerospace. A more stable Pentagon budget outlook and low interest rates have spurred two big deals in the sector this year. Raytheon Co. this week announced plans to take control of cyberspecialist Websense Inc. for $1.7 billion, and Harris Corp. is buying Exelis Inc. for around $4.5 billion.

Lockheed reported profits of $878 million in the March quarter compared with $933 million a year earlier, with per-share earnings dipping to $2.74 from $2.87 a share. Revenue fell 5.1% to $10.1 billion.

Profits beat the $2.50 consensus among analysts, driven by higher margins and a better-than-expected performance from its space unit.

For the year, Lockheed raised its earnings guidance range by 5 cents to $10.85 to $11.15 a share.

Lockheed shares were recently down 0.2% at $196.43.

Lisa Beilfuss contributed to this article.

Write to Doug Cameron at doug.cameron@wsj.com

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